diff --git a/internal/class.h b/internal/class.h
index 4a3e0afafb..b6da66a61d 100644
--- a/internal/class.h
+++ b/internal/class.h
@@ -658,6 +658,7 @@ RCLASS_SET_REFINED_CLASS(VALUE klass, VALUE refined)
 static inline rb_alloc_func_t
 RCLASS_ALLOCATOR(VALUE klass)
 {
+    RUBY_ASSERT(RB_TYPE_P(klass, T_CLASS) || RB_TYPE_P(klass, T_ICLASS));
     if (RCLASS_SINGLETON_P(klass) || RB_TYPE_P(klass, T_ICLASS)) {
         return 0;
     }