diff --git a/src/Ulid.php b/src/Ulid.php
index de3ac4e..d2c52ca 100644
--- a/src/Ulid.php
+++ b/src/Ulid.php
@@ -134,7 +134,7 @@ private function decodeTime(string $time): int
         $carry = 0;
 
         foreach ($timeChars as $index => $char) {
-            if (($encodingIndex = strrpos(static::ENCODING_CHARS, $char)) === false) {
+            if (($encodingIndex = strripos(static::ENCODING_CHARS, $char)) === false) {
                 throw new InvalidUlidStringException('Invalid ULID character: ' . $char);
             }
 
diff --git a/tests/UlidTest.php b/tests/UlidTest.php
index 231814c..c636afd 100644
--- a/tests/UlidTest.php
+++ b/tests/UlidTest.php
@@ -88,5 +88,6 @@ public function testCreatesFromStringWithTrailingNewLine(): void
     public function testConvertsToTimestamp(): void
     {
         $this->assertEquals(1561622862, Ulid::fromString('0001EH8YAEP8CXP4AMWCHHDBHJ')->toTimestamp());
+        $this->assertEquals(1561622862, Ulid::fromString('0001eh8yaep8cxp4amwchhdbhj', true)->toTimestamp());
     }
 }