@SallyMeander:I think it could be a missing model binder to convert an array.
Btw, my company's MVC web application needs to add 2 custom derivatives of DefaultModelBinder to make it work for us (because it won't handle number with thousand seperator). Maybe you should write your own ModelBinder too to work around this.
I'm not of the opinion that input format parsing and acceptance should be done with a ModelBinder - you can do this directly at the Model with property attributes. Generally speaking you never need to derive a new ModelBinder.