Here are the notes for upgrading nmdb from one version to another.

While normally nothing should be done, sometimes things change in incompatible
ways. Here's the listing for the releases, you should always check it before
upgrading.

0.20 -> 0.21
 * nmdb_get() and nmdb_cache_get() now return -1 instead of 0 when there is a
   missing key. This was done to allow setting a value to be "" (no data). All
   bindings have been upgraded, so you only need to worry if you used the C
   library.