diff --git a/cc/gen.go b/cc/gen.go
index c9f45ee..7516b28 100644
--- a/cc/gen.go
+++ b/cc/gen.go
@@ -98,6 +98,7 @@
 	}
 
 	cmd.Text("BISON_PKGDATADIR=prebuilts/build-tools/common/bison").
+		FlagWithInput("M4=", ctx.Config().PrebuiltBuildTool(ctx, "m4")).
 		Tool(ctx.Config().PrebuiltBuildTool(ctx, "bison")).
 		Flag("-d").
 		Flags(flags).