How I can solve the error in maple which is ( (in PDEtools:-DeterminingPDE) expected the number of infinitesimals (4) to be equal to the sum of the number of independent (2) and dependent (1) variables; received: 4 <> 2 + 1)

I wrote the following orders

With(GeM):
gem_decl_vars(indeps=[t,x], deps=[u(t,x)]);
gem_decl_eqs([diff(u(t,x),t,t)+a∗diff(u(t,x),x,x)+b∗
u(t,x)+c∗(u(t,x))^3=0],solve_for=[diff(u(t,x),t,t)])

det_eqs:=gem_conslaw_det_eqs([t,x,u(t,x),diff(u(t,x),
t),diff(u(t,x),x)]):
CL_multipliers:=gem_conslaw_multipliers();
simplified_eqs:=DEtools[rifsimp](det_eqs,
CL_multipliers, mindim=1)

in maple 17, but the program cannot calculate and gives me an error. Does anyone help me to solve this problem.

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