diff --git a/lib/Stampie/Mailer.php b/lib/Stampie/Mailer.php
index 1fda423..cb72328 100644
--- a/lib/Stampie/Mailer.php
+++ b/lib/Stampie/Mailer.php
@@ -182,7 +182,7 @@ protected function buildIdentityString($identities)
 *@return array
      * @see \Stampie\Utils\AttachmentUtils::processAttachments
      */
-    protected function processAttachments(array $attachments, callable $callback)
+    protected function processAttachments(array $attachments, $callback)
     {
         return AttachmentUtils::processAttachments($attachments, $callback);
     }
diff --git a/lib/Stampie/Mailer/MailGun.php b/lib/Stampie/Mailer/MailGun.php
index e002a8a..b71746c 100644
--- a/lib/Stampie/Mailer/MailGun.php
+++ b/lib/Stampie/Mailer/MailGun.php
@@ -137,7 +137,7 @@ protected function handle(ResponseInterface $response)
     /**
      * {@inheritdoc}
      */
-    protected function processAttachments(array $attachments, callable $callback){
+    protected function processAttachments(array $attachments, $callback){
         return array_values(parent::processAttachments($attachments, $callback));
     }
 }
diff --git a/lib/Stampie/Mailer/Mandrill.php b/lib/Stampie/Mailer/Mandrill.php
index c297f45..a6f6984 100644
--- a/lib/Stampie/Mailer/Mandrill.php
+++ b/lib/Stampie/Mailer/Mandrill.php
@@ -141,7 +141,7 @@ protected function getAttachmentContent(AttachmentInterface $attachment){
     /**
      * {@inheritdoc}
      */
-    protected function processAttachments(array $attachments, callable $callback)
+    protected function processAttachments(array $attachments, $callback)
     {
         // Strip keys
         return array_values(parent::processAttachments($attachments, $callback));
diff --git a/lib/Stampie/Mailer/Postmark.php b/lib/Stampie/Mailer/Postmark.php
index 7e7e02e..0592bca 100644
--- a/lib/Stampie/Mailer/Postmark.php
+++ b/lib/Stampie/Mailer/Postmark.php
@@ -120,7 +120,7 @@ protected function getAttachmentContent(AttachmentInterface $attachment){
     /**
      * {@inheritdoc}
      */
-    protected function processAttachments(array $attachments, callable $callback){
+    protected function processAttachments(array $attachments, $callback){
         // Strip keys
         return array_values(parent::processAttachments($attachments, $callback));
     }
diff --git a/lib/Stampie/Util/AttachmentUtils.php b/lib/Stampie/Util/AttachmentUtils.php
index cf4ec55..06ae57a 100644
--- a/lib/Stampie/Util/AttachmentUtils.php
+++ b/lib/Stampie/Util/AttachmentUtils.php
@@ -20,8 +20,12 @@ abstract class AttachmentUtils
      *
      * @throws \InvalidArgumentException
      */
-    public static function processAttachments(array $attachments, callable $callback)
+    public static function processAttachments(array $attachments, $callback)
     {
+        if (!is_callable($callback)) {
+            throw new \InvalidArgumentException('Callback must be callable');
+        }
+
         $processed    = array();
 
         foreach ($attachments as $attachment) {
diff --git a/tests/Stampie/Tests/Mailer/MailGunTest.php b/tests/Stampie/Tests/Mailer/MailGunTest.php
index 6e2c783..742a81a 100644
--- a/tests/Stampie/Tests/Mailer/MailGunTest.php
+++ b/tests/Stampie/Tests/Mailer/MailGunTest.php
@@ -45,9 +45,10 @@ public function testHeaders()
 
     public function testGetFiles(){
         $self  = $this; // PHP5.3 compatibility
-        $token = self::SERVER_TOKEN;
-        $buildMocks = function($attachments, &$invoke) use($self, $token){
-            $mailer = $self->getMock('\\Stampie\\Mailer\\MailGun', null, array($self->adapter, $token));
+        $adapter = $this->adapter;
+        $token   = self::SERVER_TOKEN;
+        $buildMocks = function($attachments, &$invoke) use($self, $adapter, $token){
+            $mailer = $self->getMock('\\Stampie\\Mailer\\MailGun', null, array($adapter, $token));
 
             // Wrap protected method with accessor
             $mirror = new \ReflectionClass($mailer);
diff --git a/tests/Stampie/Tests/Mailer/MandrillTest.php b/tests/Stampie/Tests/Mailer/MandrillTest.php
index a41dd0f..e890e61 100644
--- a/tests/Stampie/Tests/Mailer/MandrillTest.php
+++ b/tests/Stampie/Tests/Mailer/MandrillTest.php
@@ -177,9 +177,10 @@ public function testFormatAttachments()
 
     public function testGetFiles(){
         $self  = $this; // PHP5.3 compatibility
-        $token = self::SERVER_TOKEN;
-        $buildMocks = function($attachments, &$invoke) use($self, $token){
-            $mailer = $self->getMock('\\Stampie\\Mailer\\Mandrill', null, array($self->adapter, $token));
+        $adapter = $this->adapter;
+        $token   = self::SERVER_TOKEN;
+        $buildMocks = function($attachments, &$invoke) use($self, $adapter, $token){
+            $mailer = $self->getMock('\\Stampie\\Mailer\\Mandrill', null, array($adapter, $token));
 
             // Wrap protected method with accessor
             $mirror = new \ReflectionClass($mailer);
diff --git a/tests/Stampie/Tests/Mailer/PostmarkTest.php b/tests/Stampie/Tests/Mailer/PostmarkTest.php
index e27179e..cc6e274 100644
--- a/tests/Stampie/Tests/Mailer/PostmarkTest.php
+++ b/tests/Stampie/Tests/Mailer/PostmarkTest.php
@@ -151,9 +151,10 @@ public function testFormatAttachments()
 
     public function testGetFiles(){
         $self  = $this; // PHP5.3 compatibility
-        $token = self::SERVER_TOKEN;
-        $buildMocks = function($attachments, &$invoke) use($self, $token){
-            $mailer = $self->getMock('\\Stampie\\Mailer\\Postmark', null, array($self->adapter, $token));
+        $adapter = $this->adapter;
+        $token   = self::SERVER_TOKEN;
+        $buildMocks = function($attachments, &$invoke) use($self, $adapter, $token){
+            $mailer = $self->getMock('\\Stampie\\Mailer\\Postmark', null, array($adapter, $token));
 
             // Wrap protected method with accessor
             $mirror = new \ReflectionClass($mailer);
diff --git a/tests/Stampie/Tests/Mailer/SendGridTest.php b/tests/Stampie/Tests/Mailer/SendGridTest.php
index 93309f2..131e531 100644
--- a/tests/Stampie/Tests/Mailer/SendGridTest.php
+++ b/tests/Stampie/Tests/Mailer/SendGridTest.php
@@ -180,9 +180,10 @@ public function testFormatAttachments()
 
     public function testGetFiles(){
         $self  = $this; // PHP5.3 compatibility
-        $token = self::SERVER_TOKEN;
-        $buildMocks = function($attachments, &$invoke) use($self, $token){
-            $mailer = $self->getMock('\\Stampie\\Mailer\\SendGrid', null, array($self->adapter, $token));
+        $adapter = $this->adapter;
+        $token   = self::SERVER_TOKEN;
+        $buildMocks = function($attachments, &$invoke) use($self, $adapter, $token){
+            $mailer = $self->getMock('\\Stampie\\Mailer\\SendGrid', null, array($adapter, $token));
 
             // Wrap protected method with accessor
             $mirror = new \ReflectionClass($mailer);