Processing not working when JSON has only one row with a null value

Here's the situation: I have a table that displays assignments, but the end-date of the assignment can be null.
It works fine when there are assignments with end-dates, however as soon as my data only contains a single record, and the end-date is null, it doesn't work anymore.
I'm using sAjaxSource to get the JSON data and when that happens I get the following JS-Error:

"Uncaught TypeError: Cannot read property 'length' of null" in jquery.dataTables.js:2649

Here is my Table:

projectMemberTable = $('#project_member_entries').dataTable({
	"bProcessing" : true,
	"sAjaxDataProp" : "data",
	"sAjaxSource" : baseURL + memberURL + '/all',
	"aoColumns" : [ {
		"mData" : "id",
		"bSearchable" : false,
		"bVisible" : false
	}, {
		"mData" : "user",
		"bSearchable" : false,
		"bVisible" : false
	}, {
		"mData" : "userName"
	}, {
		"mData" : "role",
		"mRender" : function(data, type, full) {
			return projectRoles[data];
		}
	}, {
		"mData" : "startDate",
		"mRender" : dataTableDate
	}, {
		"mData" : "endDate",
		"mRender" : dataTableDate
	}, {
		"mData" : null,
		"bSearchable" : false,
		"bSortable" : false,
		"fnRender" : function(oObj) {
			return '<a class="project_member_editer"><img src="' + baseURL + 'resources/images/icons/edit_16_blue.png" alt="Edit" />';
		}
	}, {
		"mData" : null,
		"bSearchable" : false,
		"bSortable" : false,
		"fnRender" : function(oObj) {
			return '<a  class="project_member_deleter"><img src="' + baseURL + 'resources/images/icons/delete_16_red.png" alt="Delete" />';
		}
	} ]
});

And here is the exact JSON that causes this problem:

{
  "data" : [ {
    "id" : 128,
    "user" : 5,
    "project" : 127,
    "startDate" : "2012-12-22T15:13",
    "endDate" : null,
    "userName" : "Benjamin Boss",
    "role" : "LEADER"
  } ]
}

I've tried to debug it, but I can't seem to find the reason. :/