SharePoint 2013 GetPermissionCollection() Asserts with 401: Unauthorized

Good afternoon all;

I am calling the GetPermissionCollection() web service function and passing in user credentials for a user with Full Control.

I get the same assert if I set UseDefaultCredentials to true.

The call is asserting with the following message: "The request failed with HTTP status 401: Unauthorized."

If I call the same web service function on a SharePoint 2010 server, the request does not assert.

I have looked at the permissions on SharePoint 2013 and don't see anything specific to this issue.

Let me know if I need to provide more info.