diff --git a/executable_semantics/syntax/BUILD b/executable_semantics/syntax/BUILD
index 5bcaf716ad72..9f22bebe3960 100644
--- a/executable_semantics/syntax/BUILD
+++ b/executable_semantics/syntax/BUILD
@@ -54,6 +54,7 @@ genrule(
     outs = [
         "parser.cpp",
         "parser.h",
+        "parser.output",
     ],
     cmd = "M4=$(M4) $(BISON) " +
           "--output=$(location parser.cpp) " +