gnuplot development

Hi there,

I have a curve in a datafile that is supposed to be the weighted sum of other two curves. I wish to find the coefficients a,b for such sum so that curve1=curve2*a+curve3*b.
I tried with the following fit:

fit 0.0 "< paste curve1.dat curve2.dat curve3.dat" u 1:($2-$4*a-$6*b) via a,b

but it sticks with the "Singular matrix" error.
Is there any workaround for that?

Thanks in advance,
Roberto