Singular

LIB "finitediff.lib";
list D="Ut","Ux","Uy","U";
list V="t","x","y";
list P="a","b";
setinitials(V,D,P);
matrix M[2][2]=0,-a,-a,0;
list Mat=unitmat(2),M;
list Appr=forward(Ut,U,t),forward(Ux,U,x);
//matrixsystem(Mat,Appr);
//timestep(_);
fouriersystem(Mat,Appr);
==> [1]:
==>    [1]:
==>       [1]:
==>          _[1,1]=(T)
==>          _[1,2]=0
==>          _[2,1]=0
==>          _[2,2]=(T)
==>       [2]:
==>          _[1,1]=1
==>          _[1,2]=(I*Sx*a*dt+Cx*a*dt-a*dt)/(dx)
==>          _[2,1]=(I*Sx*a*dt+Cx*a*dt-a*dt)/(dx)
==>          _[2,2]=1
==>    [2]:
==>       [1]:
==>          _[1]=(T)
==>       [2]:
==>          _[1]=(-I*Sx*a*dt-Cx*a*dt+a*dt+dx)/(dx)
==>          _[2]=(I*Sx*a*dt+Cx*a*dt-a*dt+dx)/(dx)
==> [2]:
==>    [1]:
==>       [1]:
==>          (T^2)
==>    [2]:
==>       [1]:
==>          (-2*Cx*a^2*dt^2-2*Cx*a*dt*dx+2*a^2*dt^2+2*a*dt*dx+dx^2)/(dx^2)
==>       [2]:
==>          (-2*Cx*a^2*dt^2+2*Cx*a*dt*dx+2*a^2*dt^2-2*a*dt*dx+dx^2)/(dx^2)
qepcadsystem(_[2]);
==> 0
==> 0
==> 0
==> sh: qepcad: Kommando nicht gefunden.
==> sh: qepcadfilter.pl: Kommando nicht gefunden.
==> 32512
==> 
==>    ? Try manually
==>    ? leaving finitediff.lib::qepcadsystem