```python
LADDER = (("m", 1000), ("cm", 10), ("mm", 1))


def split_length(mm: int) -> str:
    if not isinstance(mm, (int, float)) or mm < 0:
        raise ValueError("Length must be a non-negative integer.")
    
    units = []
    for unit, value in LADDER:
        count = mm // value
        if count > 0:
            units.append(f"{count}{unit}")
        mm %= value
    
    return " ".join(units)
```