subclass's __instanceDict.__index should be a table if there is no custom __index

middleclass.lua

@@ -147,8 +147,10 @@ local DefaultMixin = {
       local subclass = _createClass(name, self)
 
       for methodName, f in pairs(self.__instanceDict) do
+        if not (methodName == "__index" and type(f) == "table") then
           _propagateInstanceMethod(subclass, methodName, f)
         end
+      end
       subclass.initialize = function(instance, ...) return self.initialize(instance, ...) end
 
       self.subclasses[subclass] = true