Unable to issue warnings using Steam64ID

(Note: we're currently using Carbon in place of Oxide)

When attempting to issue a player warning using their Steam64ID (rather than their username) the warn fails to save and the following error is logged in the console:

08.10 00:40:56 [INFO] [SmartWarnings] [cmdWarnGUI] An unexpected error occurred 76560000000000000 executed command warn
08.10 00:40:56 [INFO] args: 76560000000000000 TC griefing is not allowed on this server
08.10 00:40:56 [INFO] System.NullReferenceException: Object reference not set to an instance of an object
08.10 00:40:56 [INFO] at Oxide.Plugins.SmartWarnings.WarnPlayer (Oxide.Core.Libraries.Covalence.IPlayer targetPlayer, Oxide.Core.Libraries.Covalence.IPlayer adminPlayer, System.String preset) [0x00399] in /server/carbon/plugins/SmartWarnings.cs:596
08.10 00:40:56 [INFO] at Oxide.Plugins.SmartWarnings.cmdWarnGUI (Oxide.Core.Libraries.Covalence.IPlayer player, System.String cmd, System.String[] args) [0x003fa] in /server/carbon/plugins/SmartWarnings.cs:547