--- a/python/mozbuild/mozbuild/action/file_generate.py
+++ b/python/mozbuild/mozbuild/action/file_generate.py
@@ -59,17 +59,17 @@ def main(argv):
     method = args.method_name
     if not hasattr(module, method):
         print('Error: script "{0}" is missing a {1} method'.format(script, method),
               file=sys.stderr)
         return 1
 
     ret = 1
     try:
-        with FileAvoidWrite(args.output_file) as output:
+        with FileAvoidWrite(args.output_file, mode='rb') as output:
             ret = module.__dict__[method](output, *args.additional_arguments, **kwargs)
             # The following values indicate a statement of success:
             #  - a set() (see below)
             #  - 0
             #  - False
             #  - None
             #
             # Everything else is an error (so scripts can conveniently |return