diff --git a/class.c b/class.c
index 28743a229e..572275f5ce 100644
--- a/class.c
+++ b/class.c
@@ -1109,7 +1109,8 @@ rb_include_module(VALUE klass, VALUE module)
     if (RB_TYPE_P(klass, T_MODULE)) {
         rb_subclass_entry_t *iclass = RCLASS_SUBCLASSES(klass);
         // skip the placeholder subclass entry at the head of the list
-        if (iclass && !iclass->klass) {
+        if (iclass) {
+            RUBY_ASSERT(!iclass->klass);
             iclass = iclass->next;
         }