diff --git a/tool/grind.dart b/tool/grind.dart
index 71d4e92..a63a642 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -34,4 +34,4 @@
       positionals: [new Positional(valueHelp: valueHelp)]));
 }
 
-typedef String Parser(String);
+typedef void Parser(String s);