diff --git a/clvm/op_utils.py b/clvm/op_utils.py
index bc409a8e..ad3a20a0 100644
--- a/clvm/op_utils.py
+++ b/clvm/op_utils.py
@@ -1,8 +1,11 @@
 import types
-from typing import Callable, Dict
+from typing import Callable, Dict, Optional
 
 
-def operators_for_dict(keyword_to_atom: Dict, op_dict: Dict[str, Callable], op_name_lookup: Dict = {}) -> Dict:
+def operators_for_dict(keyword_to_atom: Dict, op_dict: Dict[str, Callable], op_name_lookup: Optional[Dict] = None) -> Dict:
+    if op_name_lookup is None:
+        op_name_lookup = {}
+
     d = {}
     for op in keyword_to_atom.keys():
         op_name = "op_%s" % op_name_lookup.get(op, op)
@@ -12,5 +15,7 @@ def operators_for_dict(keyword_to_atom: Dict, op_dict: Dict[str, Callable], op_n
     return d
 
 
-def operators_for_module(keyword_to_atom: Dict, mod: types.ModuleType, op_name_lookup: Dict = {}) -> Dict:
+def operators_for_module(keyword_to_atom: Dict, mod: types.ModuleType, op_name_lookup: Optional[Dict] = None) -> Dict:
+    if op_name_lookup is None:
+        op_name_lookup = {}
     return operators_for_dict(keyword_to_atom, mod.__dict__, op_name_lookup)
diff --git a/clvm/serialize.py b/clvm/serialize.py
index 04fb4a6d..77cb9af9 100644
--- a/clvm/serialize.py
+++ b/clvm/serialize.py
@@ -43,7 +43,7 @@ def sexp_to_byte_iterator(sexp: SExp) -> typing.Iterator[bytes]:
     todo_stack = [sexp]
     while todo_stack:
         sexp = todo_stack.pop()
-        pair = sexp.as_pair()
+        pair = sexp.pair
         if pair:
             yield bytes([CONS_BOX_MARKER])
             todo_stack.append(pair[1])