diff --git a/src/Registry.php b/src/Registry.php
index d15b3e4..fc21278 100644
--- a/src/Registry.php
+++ b/src/Registry.php
@@ -40,7 +40,7 @@ public function addAuxDataType(ExtensionInterface $extension, ?string $alias = n
     {
         $type = $extension->getAuxDataType();
         if (array_key_exists($type, $this->auxDataTypes)) {
-            if (!$this->auxDataTypes[$type] instanceof $extension) {
+            if (!$extension instanceof $this->auxDataTypes[$type]) {
                 throw new ExtensionException('Extension already registered');
             }
         }