# Troubleshoot FHIR writeback errors

You may run into errors when sending writeback messages to the Redox FHIR sandbox. Review these common errors and recommendations for fixing them. Or, [review writeback best practices](https://docs.redoxengine.com/permalink/kgqgY4TVw4zrc58aV51rj/#writeback-tips) to preemptively prevent errors.

> **Writeback not supported for Redox patients**
>
> You can only perform writeback requests for test patient records you’ve created, not Redox test patients.

| **Error** | **Notes** |
| --- | --- |
| `reference target(s) not found: <resource>` | Your payload may include one or more <u>reference</u> values to resources that don't exist within the Redox FHIR sandbox.  Make sure the included reference values are valid FHIR IDs and not generic values. Remember that our FHIR API specs have generic values to show the schema. They don't contain sample requests for you to copy/paste. |
| `resource id mismatch, id of existing resource: <resource id>, id from request: <other resource id>` | The bundle in your payload may include a reference to a resource that already exists within the FHIR sandbox, but you're using a different resource ID.  We recommend not including resource IDs in a writeback message. Instead, use the `fullURL`, especially if resource IDs aren't known to the originating FHIR server. [Learn more about handling FHIR IDs](/implementation-guide/handle-fhir-identifiers-and-match-patient-data). You may see this error if you're copying/pasting examples from our FHIR API schemas. We use generic values, not UUIDs, in our examples. For data on demand, you should be using UUIDs. Update the value to what exists in the FHIR sandbox, or remove the ID and use the `fullURL` instead. |
| `unknown field: \"<field name>\`" | The FHIR server isn't able to parse a value in your payload.  This typically means one of your values isn't FHIR conformant. Use a FHIR validator like one of these below to make sure your payload is FHIR conformant. [HL7 FHIR validator](https://validator.fhir.org/) [Inferno resource validator](https://inferno.healthit.gov/validator/) |
| `expected binary data: found <provided data>` | There's a mismatch between the data type provided in a specific field and what the FHIR server expects.  This error specifically indicates that the FHIR server expects a binary data string but instead received a different data type. |