I believe this is a new error since the upgrade to 2.5.2.

2x Guildbook\Guildbook-4.96523.lua:586: bad argument #2 to 'format' (string expected, got nil)
[string "=[C]"]: in function format' [string "@Guildbook\Guildbook-4.96523.lua"]:586: in function SetCharacter'
[string "@Guildbook\Guildbook-4.96523.lua"]:1853: in function ForceRosterListviewRefresh' [string "@Guildbook\Guildbook-4.96523.lua"]:1825: in function LoadCharacters'
[string "@Guildbook\Guildbook-4.96523.lua"]:1718: in function `ParseGuildRoster'
[string "@Guildbook\Core.lua"]:2042: in function <Guildbook\Core.lua:2040>

Locals:
(*temporary) = "Mobile-%s"
(*temporary) = nil
(*temporary) = "string expected, got nil"