fnServerData issue

Hi DataTables,

I have an issues where my returned json is failing.

basically its gets to the function as below and fails on

Microsoft JScript runtime error: Unable to get value of the property 'length': object is null or undefined

/* if there is an ajax source load the data */
			if ( oSettings.sAjaxSource !== null && !oSettings.oFeatures.bServerSide )
			{
				var aoData = [];
				_fnServerParams( oSettings, aoData );
				oSettings.fnServerData.call( oSettings.oInstance, oSettings.sAjaxSource, aoData, function(json) {
					var aData = (oSettings.sAjaxDataProp !== "") ?
					 	_fnGetObjectDataFn( oSettings.sAjaxDataProp )(json) : json;
		
					/* Got the data - add it to the table */
					for ( i=0 ; i<aData.length ; i++ )
					{
						_fnAddData( oSettings, aData[i] );
					}
					
					/* Reset the init display for cookie saving. We've already done a filter, and
					 * therefore cleared it before. So we need to make it appear 'fresh'
					 */
					oSettings.iInitDisplayStart = iAjaxStart;
					
					if ( oSettings.oFeatures.bSort )
					{
						_fnSort( oSettings );
					}
					else
					{
						oSettings.aiDisplay = oSettings.aiDisplayMaster.slice();
						_fnCalculateEnd( oSettings );
						_fnDraw( oSettings );
					}
					
					_fnProcessingDisplay( oSettings, false );
					_fnInitComplete( oSettings, json );
				}, oSettings );
				return;
			} 

Specifically on this section,

/* Got the data - add it to the table */
					for ( i=0 ; i<aData.length ; i++ )
					{
						_fnAddData( oSettings, aData[i] );
					}

Any help would be great

Regards

Michael