Question: matrix in singular

Dear all

How to avoid in plot "matrix in singular" in my pde system

> a1 := -2;
                                     -2
> b1 := 3;
                                      3
> c1 := -1;
                                     -1
> L := 15;
                                     15
> b := .25;
                                    0.25
> th := 1.5;
                                     1.5
> pde := diff(u(x, t), x, x, x, x, t)+(6*a1-2*(3*a1*u(x, t)^2+2*b1*u(x, t)+c1))*(diff(u(x, t), x, x, x, x))-(2*(b^2-1))*(diff(u(x, t), x, x, t)+(3*a1*u(x, t)^2+2*b1*u(x, t)+c1+6*a1*u(x, t)+2*b1)*(diff(u(x, t), x, x)))+(b^2+1)^2 = 4*b*(b^2+1)^2*Heaviside(u(x, t)-th);
/ d  / d  / d  / d  / d         \\\\\
|--- |--- |--- |--- |--- u(x, t)|||||
\ dx \ dx \ dx \ dx \ dt        /////

     /                2             \ / d  / d  / d  / d         \\\\
   + \-10 + 12 u(x, t)  - 12 u(x, t)/ |--- |--- |--- |--- u(x, t)||||
                                      \ dx \ dx \ dx \ dx        ////

            / d  / d  / d         \\\
   + 1.8750 |--- |--- |--- u(x, t)|||
            \ dx \ dx \ dt        ///

            /          2                \ / d  / d         \\                
   + 1.8750 \-6 u(x, t)  - 6 u(x, t) + 5/ |--- |--- u(x, t)|| + 1.12890625 = 
                                          \ dx \ dx        //                

  1.128906250 Heaviside(u(x, t) - 1.5)
> pde1 := diff(u(x, t), x, x, x, x, t)-v(x, t)+(6*a1-2*(3*a1*u(x, t)^2+2*b1*u(x, t)+c1))*(diff(u(x, t), x, x, x, x))-(2*(b^2-1))*(diff(u(x, t), x, x, t)+(3*a1*u(x, t)^2+2*b1*u(x, t)+c1+6*a1*u(x, t)+2*b1)*(diff(u(x, t), x, x)))+(b^2+1)^2 = 4*b*(b^2+1)^2*Heaviside(v(x, t)-th);
/ d  / d  / d  / d  / d         \\\\\          
|--- |--- |--- |--- |--- u(x, t)||||| - v(x, t)
\ dx \ dx \ dx \ dx \ dt        /////          

     /                2             \ / d  / d  / d  / d         \\\\
   + \-10 + 12 u(x, t)  - 12 u(x, t)/ |--- |--- |--- |--- u(x, t)||||
                                      \ dx \ dx \ dx \ dx        ////

            / d  / d  / d         \\\
   + 1.8750 |--- |--- |--- u(x, t)|||
            \ dx \ dx \ dt        ///

            /          2                \ / d  / d         \\                
   + 1.8750 \-6 u(x, t)  - 6 u(x, t) + 5/ |--- |--- u(x, t)|| + 1.12890625 = 
                                          \ dx \ dx        //                

  1.128906250 Heaviside(v(x, t) - 1.5)
> pde2 := diff(v(x, t), t) = th*u(x, t)-b*v(x, t);
                   d                                      
                  --- v(x, t) = 1.5 u(x, t) - 0.25 v(x, t)
                   dt                                     
> IC1 := {u(-1, t) = -.7, u(1, t) = -.7, u(x, 0) = (1*1)*cos(L*x/(12.5*Pi))*exp(-(L*x/(12.5*Pi))^2), v(-1, t) = .1, v(1, t) = .1, v(x, 0) = x^2};
    /                                 
    |                                 
   < u(-1, t) = -0.7, u(1, t) = -0.7, 
    |                                 
    \                                 

                                     /               2\                  
                  /1.200000000 x\    |  1.440000000 x |                  
     u(x, 0) = cos|-------------| exp|- --------------|, v(-1, t) = 0.1, 
                  \     Pi      /    |         2      |                  
                                     \       Pi       /                  

                                \ 
                               2| 
     v(1, t) = 0.1, v(x, 0) = x  >
                                | 
                                / 
> sys := {pde1, pde2};
 // d  / d  / d  / d  / d         \\\\\          
{ |--- |--- |--- |--- |--- u(x, t)||||| - v(x, t)
 \\ dx \ dx \ dx \ dx \ dt        /////          

     /                2             \ / d  / d  / d  / d         \\\\
   + \-10 + 12 u(x, t)  - 12 u(x, t)/ |--- |--- |--- |--- u(x, t)||||
                                      \ dx \ dx \ dx \ dx        ////

            / d  / d  / d         \\\
   + 1.8750 |--- |--- |--- u(x, t)|||
            \ dx \ dx \ dt        ///

            /          2                \ / d  / d         \\                
   + 1.8750 \-6 u(x, t)  - 6 u(x, t) + 5/ |--- |--- u(x, t)|| + 1.12890625 = 
                                          \ dx \ dx        //                

  1.128906250 Heaviside(v(x, t) - 1.5), 

   d                                      \ 
  --- v(x, t) = 1.5 u(x, t) - 0.25 v(x, t) }
   dt                                     / 
> pds1 := pdsolve(sys, IC1, numeric, time = t, range = -1 .. 1, spacestep = 1/4);
module () local INFO; export plot, plot3d, animate, value, settings; option   ; end module

> p1 := pds1:-plot(t = 0.1e-1, x = -1 .. 1, numpoints = 50, color = black);

p2 := pds1:-plot(t = .2, linestyle = 2, numpoints = 50, color = black);

p3 := pds1:-plot(t = 10, numpoints = 50, color = black, linestyle = 3, labels = [u, x]);

plots[display]({p1, p2, p3});
Error, (in pdsolve/numeric/plot) unable to compute solution for t>0.:
matrix is singular

Algis