Ajax success callback not firing if argument is null or not assign?

Ajax success callback not firing if argument is null or not assign?

I used to call web service with parameter like this and works fine.

  2. function InvokeRequest(Url, Param, SuccessCallback, ErrorCallback) {
  3.     $.ajax({
  4.         type: "POST",
  5.         url: Url,
  6.         cache: false,
  7.         contentType: "application/json; charset=utf-8",
  8.         data: Param,
  9.         dataType: "json",
  10.         success: SuccessCallback,
  11.         error: ErrorCallback
  12.     });
  13. }

Today I have a web service which do not need any parameter, so I invoke with Param = "" and I have tried = null too. 
  2. function InvokeRequest(Url, SuccessCallback, ErrorCallback) {
  3.     $.ajax({
  4.         type: "POST",
  5.         url: Url,
  6.         cache: false,
  7.         contentType: "application/json; charset=utf-8",
  8.         dataType: "json",
  9.         success: SuccessCallback,
  10.         error: ErrorCallback
  11.     });
But instead of firing the success callback, it alert the result 200 ok with the JSON result.
What do I do wrong?