diff --git a/src/Api/HttpApi.php b/src/Api/HttpApi.php
index c51bb826..358c1f88 100644
--- a/src/Api/HttpApi.php
+++ b/src/Api/HttpApi.php
@@ -50,7 +50,7 @@ abstract class HttpApi
      * @param RequestBuilder  $requestBuilder
      * @param Hydrator        $hydrator
      */
-    public function __construct($httpClient, RequestBuilder $requestBuilder, Hydrator $hydrator)
+    public function __construct(ClientInterface $httpClient, RequestBuilder $requestBuilder, Hydrator $hydrator)
     {
         $this->httpClient = $httpClient;
         $this->requestBuilder = $requestBuilder;
diff --git a/src/Api/MailingList/Member.php b/src/Api/MailingList/Member.php
index b91e949d..ee9c725f 100644
--- a/src/Api/MailingList/Member.php
+++ b/src/Api/MailingList/Member.php
@@ -120,14 +120,14 @@ public function create(
 
     /**
      * Adds multiple members (up to 1000) to the mailing list.
-     * @param  string                   $list           Address of the mailing list
-     * @param  array                    $members        Array of members, each item should be either a single string address or an array of member properties
-     * @param  bool                     $upsert         `true` to update existing members, `false` (default) to ignore duplicates
-     * @param  array                    $requestHeaders
-     * @return UpdateResponse
+     * @param string $list Address of the mailing list
+     * @param array $members Array of members, each item should be either a single string address or an array of member properties
+     * @param bool $upsert `true` to update existing members, `false` (default) to ignore duplicates
+     * @param array $requestHeaders
+     * @return MailingListUpdateResponse|null
      * @throws ClientExceptionInterface
      */
-    public function createMultiple(string $list, array $members, bool $upsert = false, array $requestHeaders = [])
+    public function createMultiple(string $list, array $members, bool $upsert = false, array $requestHeaders = []): ?MailingListUpdateResponse
     {
         Assert::stringNotEmpty($list);
         Assert::isArray($members);
@@ -156,11 +156,11 @@ public function createMultiple(string $list, array $members, bool $upsert = fals
                         if (is_array($value)) {
                             $value = json_encode($value);
                         }
-                        // We should assert that "vars"'s $value is a string.
+                        break;
+                    // We should assert that "vars"'s $value is a string.
                         // no break
                     case 'name':
                         Assert::string($value);
-
                         break;
                     case 'subscribed':
                         Assert::oneOf($value, ['yes', 'no', true, false]);
diff --git a/src/Api/Pagination.php b/src/Api/Pagination.php
index 2640adcd..970bcb38 100644
--- a/src/Api/Pagination.php
+++ b/src/Api/Pagination.php
@@ -21,10 +21,19 @@
  */
 trait Pagination
 {
+    /**
+     * @param string $path
+     * @param array $parameters
+     * @param array $requestHeaders
+     * @return ResponseInterface
+     */
     abstract protected function httpGet(string $path, array $parameters = [], array $requestHeaders = []): ResponseInterface;
 
+
     /**
      * @param class-string $className
+     * @param ResponseInterface $response
+     * @return mixed
      */
     abstract protected function hydrateResponse(ResponseInterface $response, string $className);
 
@@ -49,8 +58,9 @@ public function previousPage(PagingProvider $response): ?PagingProvider
     }
 
     /**
-     * @param  PagingProvider      $response
+     * @param PagingProvider $response
      * @return PagingProvider|null
+     * @throws ClientExceptionInterface
      */
     public function firstPage(PagingProvider $response): ?PagingProvider
     {
diff --git a/src/Api/Templates.php b/src/Api/Templates.php
index ec665141..dbfd8cf1 100644
--- a/src/Api/Templates.php
+++ b/src/Api/Templates.php
@@ -47,7 +47,7 @@ public function index(string $domain, int $limit, string $page, string $pivot, a
 
         $params = [
             'limit' => $limit,
-            'skip' => $page,
+            'page' => $page,
             'p' => $pivot,
         ];
 
diff --git a/src/Assert.php b/src/Assert.php
index 19c1b7fa..58052b85 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -20,7 +20,13 @@
  */
 final class Assert extends \Webmozart\Assert\Assert
 {
-    protected static function reportInvalidArgument($message)
+    /**
+     * @psalm-pure this method is not supposed to perform side-effects
+     * @psalm-return never
+     * @param mixed $message
+     * @return void
+     */
+    protected static function reportInvalidArgument($message): void
     {
         throw new InvalidArgumentException($message);
     }
diff --git a/src/HttpClient/HttpClientConfigurator.php b/src/HttpClient/HttpClientConfigurator.php
index e2eff912..eba1616e 100644
--- a/src/HttpClient/HttpClientConfigurator.php
+++ b/src/HttpClient/HttpClientConfigurator.php
@@ -101,6 +101,10 @@ public function createConfiguredClient(): PluginClient
         return new PluginClient($this->getHttpClient(), $plugins);
     }
 
+    /**
+     * @param bool $debug
+     * @return $this
+     */
     public function setDebug(bool $debug): self
     {
         $this->debug = $debug;
@@ -108,6 +112,10 @@ public function setDebug(bool $debug): self
         return $this;
     }
 
+    /**
+     * @param string $endpoint
+     * @return $this
+     */
     public function setEndpoint(string $endpoint): self
     {
         $this->endpoint = $endpoint;
@@ -115,11 +123,18 @@ public function setEndpoint(string $endpoint): self
         return $this;
     }
 
+    /**
+     * @return string
+     */
     public function getApiKey(): string
     {
         return $this->apiKey;
     }
 
+    /**
+     * @param string $apiKey
+     * @return $this
+     */
     public function setApiKey(string $apiKey): self
     {
         $this->apiKey = $apiKey;
@@ -127,6 +142,9 @@ public function setApiKey(string $apiKey): self
         return $this;
     }
 
+    /**
+     * @return UriFactoryInterface
+     */
     private function getUriFactory(): UriFactoryInterface
     {
         if (null === $this->uriFactory) {
@@ -136,6 +154,10 @@ private function getUriFactory(): UriFactoryInterface
         return $this->uriFactory;
     }
 
+    /**
+     * @param UriFactoryInterface $uriFactory
+     * @return $this
+     */
     public function setUriFactory(UriFactoryInterface $uriFactory): self
     {
         $this->uriFactory = $uriFactory;
@@ -143,6 +165,9 @@ public function setUriFactory(UriFactoryInterface $uriFactory): self
         return $this;
     }
 
+    /**
+     * @return ClientInterface
+     */
     private function getHttpClient(): ClientInterface
     {
         if (null === $this->httpClient) {
@@ -152,6 +177,10 @@ private function getHttpClient(): ClientInterface
         return $this->httpClient;
     }
 
+    /**
+     * @param ClientInterface $httpClient
+     * @return $this
+     */
     public function setHttpClient(ClientInterface $httpClient): self
     {
         $this->httpClient = $httpClient;
@@ -159,6 +188,9 @@ public function setHttpClient(ClientInterface $httpClient): self
         return $this;
     }
 
+    /**
+     * @return History
+     */
     public function getResponseHistory(): History
     {
         return $this->responseHistory;
diff --git a/src/HttpClient/Plugin/ReplaceUriPlugin.php b/src/HttpClient/Plugin/ReplaceUriPlugin.php
index 21cf0845..b909e287 100644
--- a/src/HttpClient/Plugin/ReplaceUriPlugin.php
+++ b/src/HttpClient/Plugin/ReplaceUriPlugin.php
@@ -29,11 +29,20 @@ final class ReplaceUriPlugin implements Plugin
      */
     private $uri;
 
+    /**
+     * @param UriInterface $uri
+     */
     public function __construct(UriInterface $uri)
     {
         $this->uri = $uri;
     }
 
+    /**
+     * @param RequestInterface $request
+     * @param callable $next
+     * @param callable $first
+     * @return mixed
+     */
     public function doHandleRequest(RequestInterface $request, callable $next, callable $first)
     {
         $request = $request->withUri($this->uri);
diff --git a/src/HttpClient/RequestBuilder.php b/src/HttpClient/RequestBuilder.php
index 7ca17154..5ee230d3 100644
--- a/src/HttpClient/RequestBuilder.php
+++ b/src/HttpClient/RequestBuilder.php
@@ -40,7 +40,9 @@ class RequestBuilder
 
     /**
      * Creates a new PSR-7 request.
-     *
+     * @param string $method HTTP method
+     * @param string $uri URI
+     * @param array $headers Request headers
      * @param array|string|null $body Request body. If body is an array we will send a as multipart stream request.
      *                                If array, each array *item* MUST look like:
      *                                array (
@@ -53,7 +55,7 @@ class RequestBuilder
     public function create(string $method, string $uri, array $headers = [], $body = null): RequestInterface
     {
         if (!is_array($body)) {
-            $stream = $this->getStreamFactory()->createStream((string) $body);
+            $stream = $this->getStreamFactory()->createStream((string)$body);
 
             return $this->createRequest($method, $uri, $headers, $stream);
         }
@@ -62,8 +64,7 @@ public function create(string $method, string $uri, array $headers = [], $body =
         foreach ($body as $item) {
             $name = $this->getItemValue($item, 'name');
             $content = $this->getItemValue($item, 'content');
-            unset($item['name']);
-            unset($item['content']);
+            unset($item['name'], $item['content']);
 
             $builder->addResource($name, $content, $item);
         }
@@ -72,7 +73,7 @@ public function create(string $method, string $uri, array $headers = [], $body =
         $boundary = $builder->getBoundary();
         $builder->reset();
 
-        $headers['Content-Type'] = 'multipart/form-data; boundary="'.$boundary.'"';
+        $headers['Content-Type'] = 'multipart/form-data; boundary="' . $boundary . '"';
 
         return $this->createRequest($method, $uri, $headers, $multipartStream);
     }
@@ -90,7 +91,7 @@ private function getRequestFactory(): RequestFactoryInterface
     }
 
     /**
-     * @param  RequestFactoryInterface $requestFactory
+     * @param RequestFactoryInterface $requestFactory
      * @return $this
      */
     public function setRequestFactory(RequestFactoryInterface $requestFactory): self
@@ -113,7 +114,7 @@ private function getStreamFactory(): StreamFactoryInterface
     }
 
     /**
-     * @param  StreamFactoryInterface $streamFactory
+     * @param StreamFactoryInterface $streamFactory
      * @return $this
      */
     public function setStreamFactory(StreamFactoryInterface $streamFactory): self
@@ -136,7 +137,7 @@ private function getMultipartStreamBuilder(): MultipartStreamBuilder
     }
 
     /**
-     * @param  MultipartStreamBuilder $multipartStreamBuilder
+     * @param MultipartStreamBuilder $multipartStreamBuilder
      * @return $this
      */
     public function setMultipartStreamBuilder(MultipartStreamBuilder $multipartStreamBuilder): self
@@ -147,10 +148,10 @@ public function setMultipartStreamBuilder(MultipartStreamBuilder $multipartStrea
     }
 
     /**
-     * @param  string           $method
-     * @param  string           $uri
-     * @param  array            $headers
-     * @param  StreamInterface  $stream
+     * @param string $method
+     * @param string $uri
+     * @param array $headers
+     * @param StreamInterface $stream
      * @return RequestInterface
      */
     private function createRequest(string $method, string $uri, array $headers, StreamInterface $stream): RequestInterface
@@ -165,14 +166,14 @@ private function createRequest(string $method, string $uri, array $headers, Stre
     }
 
     /**
-     * @param  array        $item
-     * @param  string       $key
+     * @param array $item
+     * @param string $key
      * @return mixed|string
      */
     private function getItemValue(array $item, string $key)
     {
         if (is_bool($item[$key])) {
-            return (string) $item[$key];
+            return (string)$item[$key];
         }
 
         return $item[$key];
diff --git a/src/Mailgun.php b/src/Mailgun.php
index b7649a5e..9d9f53c7 100644
--- a/src/Mailgun.php
+++ b/src/Mailgun.php
@@ -220,7 +220,7 @@ public function tags(): Api\Tag
      */
     public function webhooks(): Api\Webhook
     {
-        return new Api\Webhook($this->httpClient, $this->requestBuilder, $this->hydrator, $this->apiKey);
+        return new Api\Webhook($this->httpClient, $this->requestBuilder, $this->hydrator, $this->apiKey ?? '');
     }
 
     /**
diff --git a/src/Message/BatchMessage.php b/src/Message/BatchMessage.php
index ea16191d..4ed03be1 100644
--- a/src/Message/BatchMessage.php
+++ b/src/Message/BatchMessage.php
@@ -70,10 +70,9 @@ public function __construct(Message $messageApi, string $domain, bool $autoSend)
      *                           full_name?: string,
      *                           first?: string,
      *                           last?: string,
-     *
      * @return MessageBuilder
      * @throws MissingRequiredParameter
-     * @throws TooManyRecipients
+     * @throws TooManyRecipients|ClientExceptionInterface
      */
     protected function addRecipient(string $headerName, string $address, array $variables): MessageBuilder
     {
diff --git a/src/Message/Exceptions/LimitExceeded.php b/src/Message/Exceptions/LimitExceeded.php
index fae151dc..ae801ce0 100644
--- a/src/Message/Exceptions/LimitExceeded.php
+++ b/src/Message/Exceptions/LimitExceeded.php
@@ -15,6 +15,11 @@
 
 class LimitExceeded extends \Exception implements Exception
 {
+    /**
+     * @param string $field
+     * @param int $limit
+     * @return self
+     */
     public static function create(string $field, int $limit)
     {
         return new self(sprintf('You\'ve exceeded the maximum (%d) %s for a single message.', $limit, $field));
diff --git a/src/Message/Exceptions/MissingRequiredParameter.php b/src/Message/Exceptions/MissingRequiredParameter.php
index a4925f1c..02104aff 100644
--- a/src/Message/Exceptions/MissingRequiredParameter.php
+++ b/src/Message/Exceptions/MissingRequiredParameter.php
@@ -15,6 +15,11 @@
 
 class MissingRequiredParameter extends \Exception implements Exception
 {
+    /**
+     * @param string $parameter
+     * @param string|null $message
+     * @return self
+     */
     public static function create(string $parameter, string $message = null)
     {
         if (null === $message) {
diff --git a/src/Message/Exceptions/TooManyRecipients.php b/src/Message/Exceptions/TooManyRecipients.php
index 29f47a3f..697f386f 100644
--- a/src/Message/Exceptions/TooManyRecipients.php
+++ b/src/Message/Exceptions/TooManyRecipients.php
@@ -16,11 +16,20 @@
 
 class TooManyRecipients extends LimitExceeded implements Exception
 {
+    /**
+     * @param string $field
+     * @param int $limit
+     * @return LimitExceeded|self
+     */
     public static function create(string $field, int $limit = MessageBuilder::RECIPIENT_COUNT_LIMIT)
     {
         return new self(sprintf('You\'ve exceeded the maximum recipient count (%s) for filed "%s".', $limit, $field));
     }
 
+    /**
+     * @param int $limit
+     * @return self
+     */
     public static function whenAutoSendDisabled(int $limit = MessageBuilder::RECIPIENT_COUNT_LIMIT)
     {
         return new self(sprintf('You\'ve exceeded the maximum recipient count (%s) with autosend disabled.', $limit));
diff --git a/src/Message/MessageBuilder.php b/src/Message/MessageBuilder.php
index 604f21bd..6110bdff 100644
--- a/src/Message/MessageBuilder.php
+++ b/src/Message/MessageBuilder.php
@@ -61,8 +61,9 @@ class MessageBuilder
     ];
 
     /**
-     * @param  array  $params
-     * @param  string $key
+     * @param array $params
+     * @param string $key
+     * @param mixed $default
      * @return mixed
      */
     private function get(array $params, string $key, $default)
@@ -75,7 +76,7 @@ private function get(array $params, string $key, $default)
     }
 
     /**
-     * @param  array  $params {
+     * @param array $params {
      *                        full_name?:
      *                        string,
      *                        first?:
@@ -152,9 +153,8 @@ protected function addRecipient(string $headerName, string $address, array $vari
      *                          first?: string,
      *                          last?: string,
      *                          }
-     *
      * @return MessageBuilder
-     * @throws TooManyRecipients
+     * @throws TooManyRecipients|LimitExceeded
      */
     public function addToRecipient(string $address, array $variables = []): self
     {
@@ -174,9 +174,8 @@ public function addToRecipient(string $address, array $variables = []): self
      *                          first?: string,
      *                          last?: string,
      *                          }
-     *
      * @return MessageBuilder
-     * @throws TooManyRecipients
+     * @throws TooManyRecipients|LimitExceeded
      */
     public function addCcRecipient(string $address, array $variables = []): self
     {
@@ -197,9 +196,8 @@ public function addCcRecipient(string $address, array $variables = []): self
      *                          first?: string,
      *                          last?: string,
      *                          }
-     *
      * @return MessageBuilder
-     * @throws TooManyRecipients
+     * @throws TooManyRecipients|LimitExceeded
      */
     public function addBccRecipient(string $address, array $variables = []): self
     {
@@ -270,7 +268,8 @@ public function setTemplate(string $template): self
     }
 
     /**
-     * @param  string $headerName
+     * @param string $headerName
+     * @param mixed $headerData
      * @return $this
      */
     public function addCustomHeader(string $headerName, $headerData): self
@@ -391,7 +390,7 @@ public function addCampaignId(string $campaignId): self
             throw LimitExceeded::create('campaigns', self::CAMPAIGN_ID_LIMIT);
         }
         if (isset($this->message['o:campaign'])) {
-            array_push($this->message['o:campaign'], $campaignId);
+            $this->message['o:campaign'][] = $campaignId;
         } else {
             $this->message['o:campaign'] = [$campaignId];
         }
@@ -410,7 +409,7 @@ public function addTag(string $tag): self
         }
 
         if (isset($this->message['o:tag'])) {
-            array_push($this->message['o:tag'], $tag);
+            $this->message['o:tag'][] = $tag;
         } else {
             $this->message['o:tag'] = [$tag];
         }
@@ -483,7 +482,8 @@ public function setDeliveryTime(string $timeDate, string $timeZone = null): self
     }
 
     /**
-     * @param  string $customName
+     * @param string $customName
+     * @param mixed $data
      * @return $this
      */
     public function addCustomData(string $customName, $data): self
@@ -494,7 +494,8 @@ public function addCustomData(string $customName, $data): self
     }
 
     /**
-     * @param  string $parameterName
+     * @param string $parameterName
+     * @param mixed $data
      * @return $this
      */
     public function addCustomParameter(string $parameterName, $data): self
diff --git a/src/Model/MailingList/UpdateResponse.php b/src/Model/MailingList/UpdateResponse.php
index f7a29ba3..a2f8af8a 100644
--- a/src/Model/MailingList/UpdateResponse.php
+++ b/src/Model/MailingList/UpdateResponse.php
@@ -18,6 +18,10 @@ final class UpdateResponse implements ApiResponse
     private $message;
     private $list;
 
+    /**
+     * @param array $data
+     * @return self
+     */
     public static function create(array $data): self
     {
         $model = new self();
@@ -31,11 +35,17 @@ private function __construct()
     {
     }
 
+    /**
+     * @return string
+     */
     public function getMessage(): string
     {
         return $this->message;
     }
 
+    /**
+     * @return MailingList
+     */
     public function getList(): MailingList
     {
         return $this->list;