```python
def split_length(mm: int) -> str:
    if not isinstance(mm, int) or mm < 0:
        raise ValueError("Invalid length")

    result = []
    for unit, conversion in (("m", 1000), ("cm", 10), ("mm", 1)):
        count = mm // conversion
        if count > 0:
            result.append(f"{count}{unit}")
        mm %= conversion

    return " ".join(result) or "0mm"
```