diff --git a/voluptuous/schema_builder.py b/voluptuous/schema_builder.py
index cdeb514..89800b9 100644
--- a/voluptuous/schema_builder.py
+++ b/voluptuous/schema_builder.py
@@ -249,10 +249,12 @@ def _compile_mapping(self, schema, invalid_msg=None):
         )
 
         # Keys that may have defaults
-        all_default_keys = set(
-            key
-            for key in schema
-            if isinstance(key, Required) or isinstance(key, Optional)
+        all_default_keys = tuple(
+            {
+                key: None
+                for key in schema
+                if isinstance(key, Required) or isinstance(key, Optional)
+            }.keys()
         )
 
         _compiled_schema = {}