Add unit tests for middleware

**Problem**
When working with types, 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 the existing unit-test setup, then inspect how type-related errors are currently produced. Reproduce the relevant errors and identify the messages that need more detail. Done means middleware tests cover the improved error handling and verify the descriptive messages.

Written by the indexing model from the issue text.