diff --git a/thread.c b/thread.c
index 6ccfed0003..c15c36fda4 100644
--- a/thread.c
+++ b/thread.c
@@ -269,7 +269,7 @@ rb_thread_s_debug_set(VALUE self, VALUE val)
 #endif
 
 #ifndef fill_thread_id_str
-# define fill_thread_id_string(thid, buf) (thid)
+# define fill_thread_id_string(thid, buf) (void *)(thid)
 # define fill_thread_id_str(th) (void)0
 # define thread_id_str(th) ((void *)(th)->thread_id)
 # define PRI_THREAD_ID "p"