diff --git a/reclass/storage/__init__.py b/reclass/storage/__init__.py
index 8bb64e4..3283c09 100644
--- a/reclass/storage/__init__.py
+++ b/reclass/storage/__init__.py
@@ -36,7 +36,10 @@
     def _get_class(self, name):
         raise NotImplementedError, "Storage class not implement class entity retrieval"
 
-    def _recurse_entity(self, entity, merge_base=None, seen={}, nodename=None):
+    def _recurse_entity(self, entity, merge_base=None, seen=None, nodename=None):
+        if seen is None:
+            seen = {}
+
         if merge_base is None:
             merge_base = Entity(name='empty (@{0})'.format(nodename))