I'm trying to call a WCF web service hosted by IIS using jQuery. I can call it fine without any parameters, and I can also call it fine with a GET request that includes my parameter, but as soon as I try to send in the request via POST, the call is failing.

The web service is currently nothing but:

[OperationContract, WebInvoke]
public ValidationResultSummary TestValidateOn(
    object day)
{
    return null;
}

I've set the parameter to object, to make sure the issue isn't something with type coercion. With a breakpoint in the web service, I know the call without parameters as well as the GET call with param succeeds; in the latter the expected value is sent up.

Calling code looks like:

$.ajax({
    // type: "GET",
    // url: "../Shared/Services/DomainServices.svc/TestValidateOn?day='12/Jan/2010'",

    type: "POST",
    url: "../Shared/Services/DomainServices.svc/TestValidateOn",
    // data: "{ }", --This works if object type param, calls with null
    data: "{'day': " + selectedDate + "}",  // This fails miserably
    // data: "{'day': '" + selectedDate + "'}", --This also fails miserably
    contentType: "application/json; charset=utf-8",
    dataType: "json",
    success: function(data) {
        displayResults(data.d);
    },
    error: function(xmlHttpReq, status, errThrown) {
        displayError(xmlHttpReq, status, errThrown);
    }
});

The POST call never reaches my breakpoint, and on the client, error 500 - "Internal Server Error" - is returned.

Any help would be appreciated.