Add unit tests for models

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

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

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

No files or test entry points are named. Start by locating the parser and model code and any existing unit-test structure, then identify the error paths covered by this issue; done means tests exercise those paths and verify more descriptive error messages.

Written by the indexing model from the issue text.