diff --git a/src/Exception/PackingFailedException.php b/src/Exception/PackingFailedException.php
index 682a497..f2e6fdd 100644
--- a/src/Exception/PackingFailedException.php
+++ b/src/Exception/PackingFailedException.php
@@ -18,7 +18,7 @@ class PackingFailedException extends \RuntimeException
      */
     public static function unsupportedType($value) : self
     {
-        return new self(\sprintf('Unsupported type "%s", maybe you forgot to register the type transformer?',
+        return new self(\sprintf('Unsupported type "%s", maybe you forgot to register the type transformer or extension?',
             \is_object($value) ? \get_class($value) : \gettype($value)
         ));
     }
diff --git a/tests/Unit/PackerTest.php b/tests/Unit/PackerTest.php
index 6a536ae..9b0aaad 100644
--- a/tests/Unit/PackerTest.php
+++ b/tests/Unit/PackerTest.php
@@ -47,7 +47,9 @@ public function testPack($raw, string $packed) : void
     public function testPackThrowsExceptionOnUnsupportedType($value, string $type) : void
     {
         $this->expectException(PackingFailedException::class);
-        $this->expectExceptionMessage("Unsupported type \"$type\", maybe you forgot to register the type transformer?");
+        $this->expectExceptionMessage(
+            "Unsupported type \"$type\", maybe you forgot to register the type transformer or extension?"
+        );
 
         $this->packer->pack($value);
     }
@@ -183,7 +185,9 @@ public function testPackThrowsExceptionOnUnsupportedCustomType() : void
         $packer = $this->packer->extendWith($transformer);
 
         $this->expectException(PackingFailedException::class);
-        $this->expectExceptionMessage('Unsupported type "stdClass", maybe you forgot to register the type transformer?');
+        $this->expectExceptionMessage(
+            'Unsupported type "stdClass", maybe you forgot to register the type transformer or extension?'
+        );
 
         $packer->pack($obj);
     }