Number of arguments in FormFactor

Dear MadGraph Authors:
    I'm trying to use the second method in https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FormFactors to write form factors for some loop-induced vertices. According to the description in the link, it said that I can put as many argument as I want for the form factors. So here is what I put at beginning:

VVSSF = Lorentz(name = 'VVSSF',
                 spins = [ 3, 3, 1, 1 ],
                 structure = 'FBox(P(-1,1)*P(-1,2), P(-1,1)*P(-1,3), P(-1,2)*P(-1,3), P(-1,3)*P(-1,3), P(-1,4)*P(-1,4) ) * (Metric(1,2)*P(-1,1)*P(-1,2) - P(2,1)*P(1,2))')

However, I got the error:

TypeError : 'LexToken' object has no attribute '__getitem__'

If I remove the last argument `P(-1,4)*P(-1,4)`, no error generated.

Is there any restriction here to prevent using fifth argument?

Thanks

YONGCHENG WU