AWS Cognito User Migration - Exception during user migration

I have created userpool and trying to migrate user from RDS which invokes lambda function that returns the updated event object. but its not working for me.

I have followed as provided solution by removing below 2 fields, still not working .. :(

"desiredDeliveryMediums": "EMAIL",
 "forceAliasCreation": "false"

Here is the response object that am sending from lambda. still facing same issue - Exception during user migration

Please let me know what am missing here. Thanks in advance

def lambda_handler(event, context):
     print event
     event["response"] = {
             "userAttributes": {
                 "email": event["userName"],
                 "email_verified": "true",
             },
             "finalUserStatus": "CONFIRMED",
             "messageAction": "SUPPRESS",
             "desiredDeliveryMediums": "EMAIL",
             "forceAliasCreation": "false"
         }
     print event
 return event

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