'Fuck her with her sister' interaction error

> Error: <<run>>: bad evaluation: TypeError: Cannot read property 'partners' of undefined 1116#77080de9a
> <<run addPartner(getSlave($AS), $partner)>>
> TypeError: Cannot read property 'partners' of undefined
>     at globalThis.addPartner (eval at <anonymous> (<path>/FC_pregmod.html:10291:11), <anonymous>:206159:20)
>     at eval (eval at <anonymous> (<path>/FC_pregmod.html:10291:11), <anonymous>:1:1)
>     at <path>/FC_pregmod.html:496571:11
>     at Object.evalJavaScript (<path>/FC_pregmod.html:496572:6)
>     at MacroContext.handler (<path>/FC_pregmod.html:500678:15)
>     at Object.handler (<path>/FC_pregmod.html:498246:24)
>     at Wikifier.subWikify (<path>/FC_pregmod.html:497044:45)
>     at new Wikifier (<path>/FC_pregmod.html:496943:10)
>     at <path>/FC_pregmod.html:489809:31
>     at Array.forEach (<anonymous>)

It seems to happen with any pair of sisters, I've tried it with several pairs in several saves.