There is a bug in RMDBMapSource.m
The original source on trunk:
RMLog(@"Supported zoom range: %d - %d", minZoom, maxZoom);
RMLog(@"Supported zoom range: %f - %f", minZoom, maxZoom);
minZoom and maxZoom are float values and, therefore %f should be used. Right now the zoom levels displayed on the console are invalid and after this change the values displayed on the console are correct.