diff --git a/openslides/users/apps.py b/openslides/users/apps.py
index eff921cf9..c32b82f83 100644
--- a/openslides/users/apps.py
+++ b/openslides/users/apps.py
@@ -13,6 +13,7 @@ class UsersAppConfig(AppConfig):
 
     def ready(self):
         # Import all required stuff.
+        from . import serializers  # noqa
         from ..core.signals import post_permission_creation, permission_change
         from ..utils.rest_api import router
         from .projector import get_projector_elements
diff --git a/openslides/utils/models.py b/openslides/utils/models.py
index 4b863e497..6b185cb07 100644
--- a/openslides/utils/models.py
+++ b/openslides/utils/models.py
@@ -135,5 +135,13 @@ class RESTModelMixin:
         """
         Returns the full_data of the instance.
         """
-        serializer_class = model_serializer_classes[type(self)]
+        try:
+            serializer_class = model_serializer_classes[type(self)]
+        except KeyError:
+            # Because of the order of imports, it can happen, that the serializer
+            # for a model is not imported yet. Try to guess the name of the
+            # module and import it.
+            module_name = type(self).__module__.rsplit(".", 1)[0] + ".serializers"
+            __import__(module_name)
+            serializer_class = model_serializer_classes[type(self)]
         return serializer_class(self).data