--- a/python/mozbuild/mozbuild/frontend/reader.py
+++ b/python/mozbuild/mozbuild/frontend/reader.py
@@ -182,16 +182,17 @@ class MozbuildSandbox(Sandbox):
 
     context is a Context instance.
 
     metadata is a dict of metadata that can be used during the sandbox
     evaluation.
     """
     def __init__(self, context, metadata={}, finder=default_finder):
         assert isinstance(context, Context)
+        metadata = metadata or {}
 
         Sandbox.__init__(self, context, finder=finder)
 
         self._log = logging.getLogger(__name__)
 
         self.metadata = dict(metadata)
         exports = self.metadata.get('exports', {})
         self.exports = set(exports.keys())
@@ -1025,17 +1026,17 @@ class BuildReader(object):
                 source = fh.read()
 
             tree = ast.parse(source, full)
             Visitor().visit(tree)
 
             for name, key, value in assignments:
                 yield p, name, key, value
 
-    def read_mozbuild(self, path, config, descend=True, metadata={}):
+    def read_mozbuild(self, path, config, descend=True, metadata=None):
         """Read and process a mozbuild file, descending into children.
 
         This starts with a single mozbuild file, executes it, and descends into
         other referenced files per our traversal logic.
 
         The traversal logic is to iterate over the *DIRS variables, treating
         each element as a relative directory path. For each encountered
         directory, we will open the moz.build file located in that
@@ -1046,16 +1047,18 @@ class BuildReader(object):
 
         Arbitrary metadata in the form of a dict can be passed into this
         function. This feature is intended to facilitate the build reader
         injecting state and annotations into moz.build files that is
         independent of the sandbox's execution context.
 
         Traversal is performed depth first (for no particular reason).
         """
+        metadata = metadata or {}
+
         self._execution_stack.append(path)
         try:
             for s in self._read_mozbuild(path, config, descend=descend,
                                          metadata=metadata):
                 yield s
 
         except BuildReaderError as bre:
             raise bre