diff --git a/gc.c b/gc.c
index 44b3f6a83c..e47a4adb1f 100644
--- a/gc.c
+++ b/gc.c
@@ -8920,8 +8920,8 @@ rb_gc_writebarrier(VALUE a, VALUE b)
 {
     rb_objspace_t *objspace = &rb_objspace;
 
-    if (RGENGC_CHECK_MODE && SPECIAL_CONST_P(a)) rb_bug("rb_gc_writebarrier: a is special const");
-    if (RGENGC_CHECK_MODE && SPECIAL_CONST_P(b)) rb_bug("rb_gc_writebarrier: b is special const");
+    if (SPECIAL_CONST_P(a)) rb_bug("rb_gc_writebarrier: a is special const: %"PRIxVALUE, a);
+    if (SPECIAL_CONST_P(b)) rb_bug("rb_gc_writebarrier: b is special const: %"PRIxVALUE, b);
 
   retry:
     if (!is_incremental_marking(objspace)) {