Node location: City only saved if State is set too

In a node's location fields (using the bundled Node Location, not Location CCK), the city field does not save unless you enter a State too. There is no error message saying the State field is required. In the content type settings, both City and State are set to "Allow" and geocoding is switched on.

Is there a good rationale for not saving the City independently of the State? If so, it would be good to provide an error message so this is clearer to end users - right now it just appears as though the field is broken.

Apologies if this is a duplicate, I had a good search and couldn't find anything describing this particular issue.

Thanks, David