Passing IDictionary object to Web service

Question

In my asmx file, I have

        [WebMethod]
        [ScriptMethod]
        public void Method(IDictionary<string, CustomClass> objectOfCustomClass)
        {
           //do stuff
        }

custom class is defined as:

public class CustomClass
    {
        public string Prop1 { get; set; }
        public string Prop2 { get; set; }       
    }

All goes well and the web service is being successfully consumed from Jquery ajax method. Recently I decided to test and try and access it from the direct URL such as http://localhost/Services.asmx/Method

and I get this message

"Cannot serialize interface System.Collections.Generic.IDictionary"

This seems off. What's causing it and is it normal? I am confused - it seems like the clean way of doing things but according to Microsoft shouldn't be done, yet it works, only not when accessing the web service directly. What gives? I also read on some MS sites that you can't pass IDictionary to the web service as a parameter yet it works fine...SO can it be done or not? Can someone clarify this once and for all?