diff --git a/lib/revalidator.js b/lib/revalidator.js
index 58476b0..4f7bd1f 100644
--- a/lib/revalidator.js
+++ b/lib/revalidator.js
@@ -243,8 +243,12 @@
         type;
 
     function constrain(name, value, assert) {
-      if (schema[name] !== undefined && !assert(value, schema[name])) {
-        error(name, property, value, schema, errors);
+      if (schema[name] === undefined)
+        return;
+      var result = assert(value, schema[name]);
+      var message = typeof result === "string" ? result : null;
+      if (message !== null || !result) {
+        error(name, property, value, schema, errors, message);
       }
     }
 
@@ -420,9 +424,9 @@
     callback(true);
   }
 
-  function error(attribute, property, actual, schema, errors) {
+  function error(attribute, property, actual, schema, errors, message) {
     var lookup = { expected: schema[attribute], actual: actual, attribute: attribute, property: property };
-    var message = schema.messages && schema.messages[attribute] || schema.message || validate.messages[attribute] || "no default message";
+    var message = message || schema.messages && schema.messages[attribute] || schema.message || validate.messages[attribute] || "no default message";
     message = message.replace(/%\{([a-z]+)\}/ig, function (_, match) { return lookup[match.toLowerCase()] || ''; });
     errors.push({
       attribute: attribute,