It appears that there are some typos in that line of code. On line 1173 of globals.gd, it should probably be:
mother.preg.womb.append({id = father_id, unique = father_unique, semen = cumprod, virility = virility, day = 0,})
since this would use the substitute variables instead of trying to use indexes of the "father" argument. There could be other problems because I haven't tested it, just skimmed through the local code.