issue in dropdownFieldByIndex() when setting the value based on choiceLabels

I've copied your code into the Appian environment I am working in and it seems to work for me .The only thing I had to change was that I don't have access to rule!APN_isNotBlank:

a!localVariables(
  local!currMonth: datetext(today(),"M"),
  local!monthName: {"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"},

  a!dropdownFieldByIndex(
    label: "",
    labelPosition: "ABOVE",
    placeholder: "--Select Month--",
    choiceLabels:union(local!monthName,local!monthName),
    /*choiceValues: tointeger(union(local!monthValue,local!monthValue)),*/
    value: if(rule!GBL_isNotBlank(ri!selectedMonth),ri!selectedMonth,local!currMonth),
    saveInto:ri!selectedMonth,
    searchDisplay: "ON"
  )
)

...otherwise it's identical to what you've provided., I do not see an error.

There is a possibility that you're passing 0 in the value of ri!selectedMonth from, say, a process that has this User Interface inside a User Input Task...in which case you need to default the value to null and not 0

I've copied your code into the Appian environment I am working in and it seems to work for me .The only thing I had to change was that I don't have access to rule!APN_isNotBlank:

a!localVariables(
  local!currMonth: datetext(today(),"M"),
  local!monthName: {"Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"},

  a!dropdownFieldByIndex(
    label: "",
    labelPosition: "ABOVE",
    placeholder: "--Select Month--",
    choiceLabels:union(local!monthName,local!monthName),
    /*choiceValues: tointeger(union(local!monthValue,local!monthValue)),*/
    value: if(rule!GBL_isNotBlank(ri!selectedMonth),ri!selectedMonth,local!currMonth),
    saveInto:ri!selectedMonth,
    searchDisplay: "ON"
  )
)

...otherwise it's identical to what you've provided., I do not see an error.

There is a possibility that you're passing 0 in the value of ri!selectedMonth from, say, a process that has this User Interface inside a User Input Task...in which case you need to default the value to null and not 0

hahah i am pretty happy that we replied the same suggestion :D

Stewart Burchell: Thanks for your response. I am passing the ri!selectedMonth in UserInputTask through Process model then I am getting error. What steps should I apply to fix the issue.

Where you are mapping the pv!selectedMonth into your Interface (inside the data mapping of the User Input Task) default the value to null.

isn't the default value of a pv of type integer = 0? 

Yes. So you have to override if you want to pass this to the User Interface. Or, as an alternative, catch it in the rule input in the Interface and, if set to 0, override it to null. Or set it to the current month value if it's initialized to 0. Or any other of a dozen ways to skin a proverbial feline!