Add unit tests for types

**Problem**
When working with core, 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 core type-handling code and any existing unit-test setup. Review the current error paths and define tests for the requested descriptive messages; done means the relevant type failures are covered and the test suite passes.

Written by the indexing model from the issue text.