Invalid Datatype error when creating custom field.

Hello,

When creating a custom field expression using the following API:

LORA1APP.ORDER_SUPPLY_DEMAND_API.GET_ORDER_SUPPLY_DEMAND_TAB(CONTRACT, PART_NO, CONFIGURATION_ID, PROJECT_ID)

I receive the following error. I have tried formatting the API with TO_CHAR but no luck. Any guidance on how to resolve this?

I have also tried writing the following SQL statement to return the results field ORDER_SUPPLY_DEMAND_TYPE

SELECT ORDER_SUPPLY_DEMAND_TYPE
FROM CUSTOMER_ORDER_LINE_DEMAND
WHERE order_no = :order_no
AND line_no =:line_no
AND rel_no = :rel_no

When using the above SQL argument to create the custom field I receive the following error

Please advise, Thanks.