diff --git a/src/compiler/compiler_internal.h b/src/compiler/compiler_internal.h
index 2010634fb..a9a8b0e8c 100644
--- a/src/compiler/compiler_internal.h
+++ b/src/compiler/compiler_internal.h
@@ -358,7 +358,7 @@ typedef struct
 {
 	struct
 	{
-		bool is_builtin;
+		bool is_builtin : 1;
 		bool attr_weak : 1;
 		bool attr_noreturn : 1;
 		bool attr_inline : 1;