```python
from typing import List, Dict

def tags_of(line: str) -> List[str]:
    if not isinstance(line, str):
        raise ValueError("Input must be a string")
    
    return [tag.strip() for tag in line.split(',') if tag.strip()]

def tag_index(lines: List[str]) -> Dict[str, List[int]]:
    if not all(isinstance(line, str) for line in lines):
        raise ValueError("All input must be strings")
    
    tag_dict = {}
    for i, line in enumerate(lines):
        tags = tags_of(line)
        for tag in tags:
            if tag in tag_dict:
                tag_dict[tag].append(i)
            else:
                tag_dict[tag] = [i]
    
    return tag_dict
```