Cannot update value of length 8 by assigning value of length 6

a!dropdownField(
label: "POL",
labelPosition: "ABOVE",
placeholder: "--- Select a Value ---",
choiceLabels: local!polDetails.pol,
choiceValues: local!polDetails.id,
value: local!pol,
saveInto: {
local!psl,
a!save(local!selectedSubPol, null)
},
searchDisplay: "AUTO",
                         
                          validations: {}
                        ),
a!multipleDropdownField(
  label: "Sub - POL",
  labelPosition: "ABOVE",
  placeholder: "--- Select Values ---",
  choiceLabels: local!subPol.subPol,
  choiceValues: local!subPol.id,
  value: if(a!isNotNullOrEmpty(local!pol),local!subPol.id,local!selectedsubPol),
  saveInto: {
    local!selectedsubPol,
    a!save(local!selectedsubPol, null)
  })

error is coming when i am un-selecting the default selected values . can somebody help me on this