--- a/python/mozbuild/mozbuild/frontend/reader.py
+++ b/python/mozbuild/mozbuild/frontend/reader.py
@@ -197,17 +197,18 @@ class MozbuildSandbox(Sandbox):
         if key in self.subcontext_types:
             return self._create_subcontext(self.subcontext_types[key])
         if key in self.templates:
             return self._create_template_function(self.templates[key])
         return Sandbox.__getitem__(self, key)
 
     def __setitem__(self, key, value):
         if key in self.special_variables or key in self.functions or key in self.subcontext_types:
-            raise KeyError()
+            raise KeyError('Cannot set "%s" because it is a reserved keyword'
+                           % key)
         if key in self.exports:
             self._context[key] = value
             self.exports.remove(key)
             return
         Sandbox.__setitem__(self, key, value)
 
     def exec_file(self, path):
         """Override exec_file to normalize paths and restrict file loading.