Columns menu - undefined coulmn

Hello,

I'm using the javascript implementation for kendo ui grid - kendo version: 2013.1.319.

I added a column menu to my grid that contains 6 column .

My columns definition is this: 

 columns: [
                     {field: 'WorkOrder().Name()', title: 'Work Order'}, 
                     {field: 'Product().Name()', title: 'Product Name'}, 
                     {field: 'Customer().Name()', title: 'Customer'},
                     {field: 'Operations', title: 'Operation'  }, 
                     {field: 'WorkOrder().PercentProduced()',  title: 'Percent Produced' }, 
                     {field: 'WorkOrder().DueDate()',type: 'date', title: 'Due Date' },
                     ],

for some reason while opening   the menu  there are  7 columns displayed when the last one's title is undefined (see the attached image).

while trying to figure out where the extra  column came from, I checked my columns attribute in the grid object using this function:

    var grid = $("#grid").data("kendoGrid");
     alert(grid.columns.length); 

the result was only 6 .

Where to look for the expendable column and how to delete it? 

Thanks

Eran