diff --git a/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py b/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
index 6f578796d..7cf44f6b3 100644
--- a/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
+++ b/test_libs/pyspec/eth2spec/utils/ssz/ssz_typing.py
@@ -73,7 +73,7 @@ class uint256(uint):
 
 # Note: importing ssz functionality locally, to avoid import loop
 
-class Container(object):
+class Container(object, metaclass=DefaultingTypeMeta):
 
     def __init__(self, **kwargs):
         cls = self.__class__