1 files changed, 1 insertions, 0 deletions
diff --git a/class.c b/class.c
index 544652a4ab..2eedb262dc 100644
--- a/class.c
+++ b/class.c
@@ -241,6 +241,7 @@ rb_include_module(klass, module)
 		return;
 	    }
 	}
+	rb_frozen_class_p(klass);
 	RCLASS(klass)->super =
 	    include_class_new(module, RCLASS(klass)->super);
 	klass = RCLASS(klass)->super;