Not sure why anyone would want a to friend an NPC.....if so, hit me up and I will be your friend.
Seriously though, a simple check for the length of the player ID would resolve this.  Ths issue is in the data file, not other plugins that use the data file data.   Great plugin, but this appears to be systemic amongst the community.  Thank you!

This plugin actually doesn't do anything with NPCs, it would be plugins that utilize it that are sending that data. The rewrite of this for uMod will include that as an option though, as it is something that is potentially a valid use.