Hi @pradilla,
I’ve tried your workflow in my self-hosted environement and got the same error. When ‘Use Header Names As JSON Paths’ activated, the code is trying to convert value to string. Since you have null value, it can not be converted.
If you have rush, you can use below workaround for now tili issue is resolved.
1 - Set your column names on Google Sheet without ‘body’ path.
2 - Transfer your input data’s body.
Hope it helps.
P.S for n8n team: I’ve created a pull request for the issue.