# error in maple's CellDecomposition command

I have simple system that I want to put in celldecomposition command of maple. I don't know why maple is giving an error here! The code is

restart;
f1 := 4*b*(5*b*(13*t^2-tau^2)*(t^2+tau^2)^6-3*a*
(5*t^6-63*t^4*tau^2+35*t^2*tau^4+7*tau^6));
f2 := 56*b*tau*(5*b*(t^2+tau^2)^6+a*(9*t^4-30*t^2*tau^2+9*tau^4));
f3 := 1-z*t;
with(RootFinding[Parametric])
m := CellDecomposition([f1 = 0, f2 = 0, f3 = 0], [t, tau, z])


and the error is:

Error, (in RootFinding:-Parametric:-CellDecomposition) The number of polynomials is smaller than the number of unknowns

The no. of polynomials is obviously the same as the no. of variables here! Any idea what is going wrong here? -dbm

-
 There are five unknowns in the three equations. – acer Jan 5 at 3:24