IIS Overwriting HTTP Response Text When HTTP Response Status set as 400

I am building an MVC 3 application with an IIS 7.5 backend. On my controller, I have action methods that allow the user to add/edit domain objects. The Action handles HTTP Post, has a return value of string which contains any validation error messages encountered during the save process. Here is an example of one Action Method:

    [HttpPost]
    public string CustomerEdit(CustomerModel customerModel)
    {
        var errorMessages = new StringBuilder();
        var serverErrors = new List<string>();

        //Map to a customer domain object
        Mapper.CreateMap<CustomerModel, Customer>();
        var customer = Mapper.Map<CustomerModel, Customer>(customerModel);

        if (customerModel.Oper == "del")
        {
            var customerIds = new List<Guid>();
            customerIds.Add(customer.Id);

            if (!_serverService.DeleteCustomers(customerIds))
            {
                errorMessages.Append("The item could not be deleted");
                Response.StatusCode = Constants.STATUS_SERVER_ERROR;
            }
        }
        else
        {
            //Validate
            if (!_serverService.ValidateCustomer(customer, out serverErrors))
            {
                foreach (var error in serverErrors)
                {
                    ModelState.AddModelError("Validation", error);
                }
            }

            //Save
            if (ModelState.IsValid)
            {
                var saveStatus = _serverService.SaveCustomer(ref customer, _id);

                if (!saveStatus)
                {
                    errorMessages.Append("The server encountered and error during Save");
                    Response.StatusCode = Constants.STATUS_SERVER_ERROR;
                }
            }
            else
            {
                errorMessages.Append(GetValidationErrors(ModelState));
                Response.StatusCode = Constants.STATUS_SERVER_ERROR;
            }