--- WebIDL.py
+++ WebIDL.py
@@ -6438,7 +6438,8 @@ class Parser(Tokenizer):
         self.parser = yacc.yacc(module=self,
                                 outputdir=outputdir,
                                 tabmodule='webidlyacc',
-                                errorlog=logger
+                                errorlog=logger,
+                                debug=False
                                 # Pickling the grammar is a speedup in
                                 # some cases (older Python?) but a
                                 # significant slowdown in others.