diff --git a/wagtail/wagtailcore/admin.py b/wagtail/wagtailcore/admin.py
index b42d7ff44..bd42c9d63 100644
--- a/wagtail/wagtailcore/admin.py
+++ b/wagtail/wagtailcore/admin.py
@@ -19,5 +19,6 @@ class GroupPagePermissionInline(admin.TabularInline):
 class GroupAdminWithPagePermissions(GroupAdmin):
     inlines = GroupAdmin.inlines + [GroupPagePermissionInline]
 
-admin.site.unregister(Group)
+if admin.site.is_registered(Group):
+    admin.site.unregister(Group)
 admin.site.register(Group, GroupAdminWithPagePermissions)