diff --git a/engine/src/flutter/sky/tools/gn b/engine/src/flutter/sky/tools/gn
index 72cc38d1584..76db10e959b 100755
--- a/engine/src/flutter/sky/tools/gn
+++ b/engine/src/flutter/sky/tools/gn
@@ -74,7 +74,7 @@ def main():
   parser.add_argument('--target-os', type=str)
   parser.add_argument('--android', dest='target_os', action='store_const', const='android')
   parser.add_argument('--ios', dest='target_os', action='store_const', const='ios')
-  parser.add_argument('--simulator', default=False)
+  parser.add_argument('--simulator', action='store_true', default=False)
 
   parser.add_argument('--goma', default=True, action='store_true')
   parser.add_argument('--no-goma', dest='goma', action='store_false')