# AN-2026-0428: Cloudflare Error 1037: Invalid rewrite rule (failed to evaluate expression)

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- id: AN-2026-0428 (version 1; cite https://breakwire.org/claim/AN-2026-0428/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://developers.cloudflare.com/support/troubleshooting/http-status-codes/cloudflare-1xxx-errors/error-1037/#:~:text=Error%201037%3A%20Invalid%20rewrite%20rule%20%28failed%20to%20evaluate%20expression%29>
- services: cloudflare-cdn, cloudflare-workers, cloudflare-pages
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Summary
```text
Cloudflare docs, Error 1037: Invalid rewrite rule (failed to evaluate expression). Common cause Resolution Error 1037: Invalid rewrite rule (failed to evaluate expression) This error indicates that the rewrite rule expression could not be evaluated. Common cause There are several causes for this error, but it can mean that one expression element contained an undefined value when it was evaluated.
```

## Statements (checkable)
0. Cloudflare docs, Error 1037: Invalid rewrite rule (failed to evaluate expression). Common cause Resolution Error 1037: Invalid rewrite rule (failed to evaluate expression) This error indicates that the rewrite rule expression could not be evaluated. Common cause There are several causes for this error, but it can mean that one expression element contained an undefined value when it was evaluated.  [✓0 ✗0]

## Fix
```text
Common cause Resolution Error 1037: Invalid rewrite rule (failed to evaluate expression) This error indicates that the rewrite rule expression could not be evaluated. Common cause There are several causes for this error, but it can mean that one expression element contained an undefined value when it was evaluated.
```

## Error signatures
- regex: `\b(error(\s+code)?|code)\s*[:=#]?\s*\[?\s*1037\b` (Cloudflare error 1037 ("error code: 1037" / "Error 1037"))

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0428/reports_

## History
- v1 2026-10-28 by wire-editor: filed from the vendor error-code catalog page opened 2026-10-28 (verbatim strings; linted)