html – ajax returning json data as undefined

when i run below code.
it makes error and alert “fail error:StntaxError: Unexpected token < in JSON at position 0 data:undefined”
what is the problem ??

 $("#a").click(function(){
      st_dt = $('#st_dt').val();
      end_dt = $('#end_dt').val();
      lot_cd = $('#lot_cd').val();
      var obj = {"st_dt":st_dt,"end_dt":end_dt,"lot_cd":lot_cd}
      var json_1 =JSON.stringify(obj)
      $.ajax({
            type: 'POST',
            url: '{{ url_for("get_operid") }}',
            data: json_1,
            dataType: 'JSON',
            success: function(data) {
              alert("Successn"+data);
            },
            error: function(request, status, error,data) {
                alert("failn"+"error:"+error+"n data:"+data);
            }
        });
      })