--- a/python/mozbuild/mozbuild/frontend/sandbox.py
+++ b/python/mozbuild/mozbuild/frontend/sandbox.py
@@ -108,20 +108,20 @@ class Sandbox(dict):
         'None': None,
         'False': False,
         'True': True,
         'sorted': alphabetical_sorted,
         'int': int,
         'set': set,
     })
 
-    def __init__(self, context, builtins=None, finder=default_finder):
+    def __init__(self, context, finder=default_finder):
         """Initialize a Sandbox ready for execution.
         """
-        self._builtins = builtins or self.BUILTINS
+        self._builtins = self.BUILTINS
         dict.__setitem__(self, '__builtins__', self._builtins)
 
         assert isinstance(self._builtins, ReadOnlyDict)
         assert isinstance(context, Context)
 
         # Contexts are modeled as a stack because multiple context managers
         # may be active.
         self._active_contexts = [context]