diff --git a/libgrust/libproc_macro_internal/proc_macro.h b/libgrust/libproc_macro_internal/proc_macro.h
index 457ec4693f6..9e142ffa79f 100644
--- a/libgrust/libproc_macro_internal/proc_macro.h
+++ b/libgrust/libproc_macro_internal/proc_macro.h
@@ -66,7 +66,7 @@ struct Bang
 
 enum ProcmacroTag
 {
-  CUSTOM_DERIVE,
+  CUSTOM_DERIVE = 0,
   ATTR,
   BANG,
 };