diff --git a/src/gc.c b/src/gc.c
index 69708e40b..470fffc5f 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -480,7 +480,18 @@ mrb_obj_alloc(mrb_state *mrb, enum mrb_vtype ttype, struct RClass *cls)
   mrb_gc *gc = &mrb->gc;
 
   if (cls) {
-    enum mrb_vtype tt = MRB_INSTANCE_TT(cls);
+    enum mrb_vtype tt;
+
+    switch (cls->tt) {
+    case MRB_TT_CLASS:
+    case MRB_TT_SCLASS:
+    case MRB_TT_MODULE:
+    case MRB_TT_ENV:
+      break;
+    default:
+      mrb_raise(mrb, E_TYPE_ERROR, "allocation failure");
+    }
+    tt = MRB_INSTANCE_TT(cls);
     if (tt != MRB_TT_FALSE &&
         ttype != MRB_TT_SCLASS &&
         ttype != MRB_TT_ICLASS &&