ASP.NET MVC Bug with Model Binding: "Specified cast is not valid" when no data provided for dictionary

It sure looks like you're running into that bug.

The bug was fixed over a year ago. Have you tried upgrading to MVC 5? That CodePlex bug report has a .cs file attached that provides a fixed dictionary binder. You can incorporate it using this in App_Start:

ModelBinder.Binders.DefaultBinder = new DefaultDictionaryBinder();

Here is the fixed binder (provided by Codeplex user richardprior):

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Web.Mvc;

/// <summary>
/// ASP.NET MVC Default Dictionary Binder, with a dictionary that doesn't bind your
/// route parameters when no values are passed
/// Adapted from: https://github.com/loune/MVCStuff/blob/master/Extensions/DefaultDictionaryBinder.cs
/// </summary>
public class FixedDictionaryBinder : DefaultModelBinder
{
    readonly IModelBinder _nextBinder;

    /// <summary>
    /// Create an instance of DefaultDictionaryBinder.
    /// </summary>
    public FixedDictionaryBinder()
        : this(null)
    {
    }

    /// <summary>
    /// Create an instance of DefaultDictionaryBinder.
    /// </summary>
    /// <param name="nextBinder">The next model binder to chain call. If null, by default, the DefaultModelBinder is called.</param>
    public FixedDictionaryBinder(IModelBinder nextBinder)
    {
        _nextBinder = nextBinder;
    }

    public override object BindModel(ControllerContext controllerContext, ModelBindingContext bindingContext)
    {
        Type modelType = bindingContext.ModelType;

        if (modelType.AssemblyQualifiedName != null && modelType.AssemblyQualifiedName.StartsWith("System.Collections.Generic.IDictionary`2"))
        {
            object result = null;

            var ga = modelType.GetGenericArguments();
            var valueBinder = Binders.GetBinder(ga[1]);

            foreach (var key in GetValueProviderKeys(controllerContext))
            {
                if (!key.StartsWith(bindingContext.ModelName + "[", StringComparison.InvariantCultureIgnoreCase))
                    continue;

                var endbracket = key.IndexOf("]", bindingContext.ModelName.Length + 1, StringComparison.InvariantCultureIgnoreCase);
                if (endbracket == -1)
                    continue;

                object dictKey;
                try
                {
                    dictKey = ConvertType(key.Substring(bindingContext.ModelName.Length + 1, endbracket - bindingContext.ModelName.Length - 1), ga[0]);
                }
                catch (NotSupportedException)
                {
                    continue;
                }

                var innerBindingContext = new ModelBindingContext
                                              {
                                                  ModelMetadata = ModelMetadataProviders.Current.GetMetadataForType(() => null, ga[1]),
                                                  ModelName = key.Substring(0, endbracket + 1),
                                                  ModelState = bindingContext.ModelState,
                                                  PropertyFilter = bindingContext.PropertyFilter,
                                                  ValueProvider = bindingContext.ValueProvider
                                              };
                var newPropertyValue = valueBinder.BindModel(controllerContext, innerBindingContext);

                if (result == null)
                    result = CreateModel(controllerContext, bindingContext, modelType);

                if (!(bool)modelType.GetMethod("ContainsKey").Invoke(result, new [] { dictKey }))
                    modelType.GetProperty("Item").SetValue(result, newPropertyValue, new [] { dictKey });
            }

            return result;
        }

        return _nextBinder != null
                   ? _nextBinder.BindModel(controllerContext, bindingContext)
                   : base.BindModel(controllerContext, bindingContext);
    }

    private static IEnumerable<string> GetValueProviderKeys(ControllerContext context)
    {
        var keys = new List<string>();
        keys.AddRange(context.HttpContext.Request.Form.Keys.Cast<string>());
        keys.AddRange(((IDictionary<string, object>)context.RouteData.Values).Keys);
        keys.AddRange(context.HttpContext.Request.QueryString.Keys.Cast<string>());
        keys.AddRange(context.HttpContext.Request.Files.Keys.Cast<string>());
        return keys;
    }

    private static object ConvertType(string stringValue, Type type)
    {
        return TypeDescriptor.GetConverter(type).ConvertFrom(stringValue);
    }
}