"Invalid Input: xxx" when submitting Request

"Invalid Input: xxx" when submitting Request

Hi,
I have a customize request template with an additional picklist auto-populate from JSON file. 
After filling up the form and upon submission of requestb I got the following message:

{"response_status":{"status_code":4000,"messages":[{"status_code":4001,"field":"udf_pick_2413","type":"failed","message":"Invalid Input"}],"status":"failed"}}

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