write records Error : Unable to write to the source due to system error.Error Details: Column 'VehicleID' cannot be null

Hello, I'm new to Appian and I'm facing an issue in Appian Step-by-Step #11 Exercise specifically in the Request maintenance process model,  it gives me this error below : 

Even though I have given the vehicle a default value in the Request Maintenance Interface and have assigned it in the store additional values node as an output.

Please let me know if you got some ideas to fix it.

Thanks in Advance!