diff --git a/bitbake/lib/bb/data_smart.py b/bitbake/lib/bb/data_smart.py
index 68d273b3a6..b1ea33fc30 100644
--- a/bitbake/lib/bb/data_smart.py
+++ b/bitbake/lib/bb/data_smart.py
@@ -296,9 +296,14 @@ class VariableHistory(object):
                 self.variables[var] = []
 
 class DataSmart(MutableMapping):
-    def __init__(self, special = COWDictBase.copy(), seen = COWDictBase.copy() ):
+    def __init__(self, special = None, seen = None ):
         self.dict = {}
 
+        if special is None:
+            special = COWDictBase.copy()
+        if seen is None:
+            seen = COWDictBase.copy()
+
         self.inchistory = IncludeHistory()
         self.varhistory = VariableHistory(self)
         self._tracking = False