Error while running event shield-generators::on_tick (ID 0)

Not sure how this happened as I have quite a mod list, but I'll try to get to the bottom of it.

 186.502 Error MainLoop.cpp:1284: Exception at tick 3028030: The mod Shield Generators (0.3.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event shield-generators::on_tick (ID 0)
real number expected got nil.
stack traceback:
        [C]: in function 'set_right_bottom'
        __shield-generators__/control.lua:902: in function <__shield-generators__/control.lua:590>