I have had no errors with this addon until recently, where helicopters will not fly, but planes still do. The server console is flooded with this message:

[ERROR] addons/wac aircraft/lua/entities/wac_hc_base/init.lua:819: bad argument #2 to '__mul' (number expected, got nil)
1. __mul - [C]:-1
2. unknown - addons/wac aircraft/lua/entities/wac_hc_base/init.lua:819

Thanks for any help.