diff --git a/src/App/GroupExtension.h b/src/App/GroupExtension.h
index 9635ef536d..810ff0d410 100644
--- a/src/App/GroupExtension.h
+++ b/src/App/GroupExtension.h
@@ -147,6 +147,7 @@ public:
  
     //override the documentobjectextension functions to make them available in python 
     bool allowObject(DocumentObject* obj)  override {
+        Base::PyGILStateLocker locker;
         Py::Object pyobj = Py::asObject(obj->getPyObject());
         EXTENSION_PROXY_ONEARG(allowObject, pyobj);