Handle edge cases in sorting

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

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

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

The issue names no files, tests, or entry points. Start by locating model sorting and the related error-handling paths, then clarify which edge cases and messages are required. Done means the agreed sorting cases are handled and the resulting errors are more descriptive.

Written by the indexing model from the issue text.