Add unit tests for types

**Problem**
When working with middleware, the error messages could be more descriptive.

**Suggestion**
Improve error handling and add detailed error messages.

**Context**
This would help with debugging and maintenance.

Start by locating the middleware implementation and its existing unit-test entry points. Review the current error paths and define completion as tests covering the relevant types and error messages becoming sufficiently descriptive for debugging.

Written by the indexing model from the issue text.