Question: Numerical partial differentials system of equations with 3 variables

Hi!

I've a problem with maple. I wonder to see the visualisation of the solutions of a system of partial equations :

with(PDEtools);
EDP := [diff(s(x, y, t), t) = -.45*i(x, y, t)*s(x, y, t), diff(i(x, y, t), t) = .45*i(x, y, t)*s(x, y, t)-.2*i(x, y, t)-.7*(diff(i(x, y, t), x)+diff(i(x, y, t), y))];
IBC := {s(0, y, t) = 1, s(x, y, 0) = f(x, y), i(x, y, 0) = g(x)};
f := proc (x) options operator, arrow; 1-g(x) end proc; g := proc (x) options operator, arrow; piecewise(x = 20, .2, 0) end proc

But when I write :

PDS := pdsolve(EDP, IBC, [s, i], numeric, time = t, range = 0 .. 100);

I become the error : Error, (in pdsolve/numeric/process_PDEs) can only numerically solve PDE with two independent variables, got {y, t, x}

Maybe do you know a solution of my problem ?
Thanks for the answers !