REST API unable to retrieve custom lookup value

Hi,

I'm trying to retrieve a list item that contains an extended lookup field. For some reason I'm keep geting this error:

The field value for field "fieldname" of type "fieldtype" cannot be serialized.

The only real customization of the field is that it has altered rendering and thus far it been working just fine.

This is my sample REST query:

jQuery.ajax(
{
	url: "/_api/web/lists/getByTitle('MyList')/items/",
	dataType: "json",
	data: "$filter=startswith(ContentTypeId, '" + parentCtId.id + "')",
	processData: false,
	headers: { ACCEPT: "application/json;odata=verbose" },
	async: false
}).done(function (data)
{
}).fail(function (e)
{
});