Erase first N items from a std::map?

Trying to write a method which deletes the first (lowest-keyed) N items from an std::map. Tried this:

void EraseNMapElements(const int numElementsToRemove){

    const int originalSize = _map.size();     
    auto eraseIter = _map.begin();
    std::advance(eraseIter, numElementsToRemove);

    _map.erase(_map.begin(), eraseIter);

    assert(_map.size() == (originalSize - numElementsToRemove)) || (0 == originalSize) || (0 == _map.size()));
}

It works when the number of elements is more than the number requested to be removed. So if I had five elements, request to delete 2, the last 3 element remain. However, if I have one element and request to erase 2, I still have one element remaining.

Is there a neat way to cover this? I could shove an IF statement around checking for numElementsToRemove greater than map.size() but there must be a better solution?