Value not in dropDown list.

I have created a custom multi select drop down which have blank list and dynamically append the options using ajax.

it is working properly in form but when i import the excel sheet and select the options from multi select drop down and click on import then error display

Value not in dropDown list.

following are my js code to use the append the dropdown list

success: function (obj) {

  for (s in obj) {

       var item = obj[s];

       $('#cust_dropdown_c')

              .append($("<option></option>")

              .attr("value",item.id)

              .text(item.name));

   }

},

Error Value not in dropDown list. Allowed values are: 

Field Name custom dropdown (cust_dropdown_c)

Value ^f3c7fd82-bd5e-30ea-280f-564db139e4e1^