LUA error when highlighting a buyable equipment item

[ManDay]

This error comes up (in the well known prompt), when I click on either "Class 2 Military Hyperdrive" or "Naval Thrusters". It does not come up for "Class 3 Military Hyperdrive", for what it's worth:

pigui/libs/equipment-outfitter.lua:27: attempt to perform arithmetic on local 'a' (a string value)

The complete backtrace from the output log is attached as error.txt