diff --git a/mapproxy/seed/config.py b/mapproxy/seed/config.py
index cb0b4d85c..a238aac6d 100644
--- a/mapproxy/seed/config.py
+++ b/mapproxy/seed/config.py
@@ -264,7 +264,7 @@ def _grids(self, caches):
         else:
             # check that all caches have the same grids configured
             last = []
-            for cache_grids in {cache.keys() for cache in caches.values()}:
+            for cache_grids in [cache.keys() for cache in caches.values()]:
                 if not last:
                     last = cache_grids
                 else: