Missing input validation could cause unexpected behavior with edge case inputs

## Bug Report

### Description
Some functions do not validate their inputs before processing, which can lead to confusing error messages or silent failures when unexpected values are passed.

### Steps to Reproduce
1. Call the affected function with `None`, an empty string, or a negative number
2. Observe that the error message is not helpful or the function silently returns incorrect results

### Expected Behavior
Functions should validate inputs early and raise descriptive errors:

```python
def process(value):
if value is None:
raise ValueError("value must not be None")
if not isinstance(value, int):
raise TypeError(f"expected int, got {type(value).__name__}")
# ... rest of function
```

### Impact
This makes debugging harder for users of the library and can hide real bugs upstream.

### Suggested Fix
Add input validation at the start of public-facing functions with clear error messages.

No affected function, file, or test is identified. Start by locating the public-facing functions involved and reproduce the cases with None, an empty string, and a negative number. Done means the affected functions reject invalid inputs with descriptive errors and no longer silently return incorrect results.

Written by the indexing model from the issue text.