diff --git a/pack.c b/pack.c
index f1f474759a..f9b1293c37 100644
--- a/pack.c
+++ b/pack.c
@@ -154,6 +154,7 @@ associated_pointer(VALUE associates, const char *t)
     UNREACHABLE_RETURN(Qnil);
 }
 
+RBIMPL_ATTR_NORETURN()
 static void
 unknown_directive(const char *mode, char type, VALUE fmt)
 {