Point shop causing errors

I have been getting odd errors. Here is the error



[ERROR] addons/pointshop-master/lua/sh_config.lua:43: attempt to call method 'IsUsergroup' (a nil value)
1. CalculateBuyPrice - addons/pointshop-master/lua/sh_config.lua:43
2. unknown - addons/pointshop-master/lua/vgui/dpointshopitem.lua:172

What is it and what does it mean?

Should be IsUserGroup() with a capitalized G.