diff --git a/server/vbv_lernwelt/course/admin.py b/server/vbv_lernwelt/course/admin.py
index d168f3b6..d4351a2c 100644
--- a/server/vbv_lernwelt/course/admin.py
+++ b/server/vbv_lernwelt/course/admin.py
@@ -66,7 +66,7 @@ class CourseSessionUserAdmin(admin.ModelAdmin):
     def formfield_for_manytomany(self, db_field, request, **kwargs):
         if db_field.name == "expert":
             if request.resolver_match.kwargs.get("object_id"):
-                object_id = int(request.resolver_match.kwargs.get("object_id"))
+                object_id = str(request.resolver_match.kwargs.get("object_id"))
                 csu = CourseSessionUser.objects.get(id=object_id)
                 kwargs["queryset"] = Circle.objects.descendant_of(
                     csu.course_session.course.coursepage