Converting Hit policy from collect to Unique

Hi,
Am having DMN file with hit policy collect and tested with RestAPI, I got response. But When I convert this file’s hit policy from collect to Unique and tested with same data, I got error like below.

{
“type”: “RestException”,
“message”: “Cannot evaluate decision NE_RULE_002:7:9f5bac9e-3b24-11ed-a839-00224868ceac: Exception while evaluating decision with key ‘null’”
}

Could you please provide any solution?