Question: How to solve system of coupled equations using maple

I am trying to solve system of coupled equations in maple, but when I execute it, maple gives a trivial answer. Here is the code:

de1:=diff(u1(x,t),x)=(i/lambda)*(p*u1(x,t)+q*u2(x,t));
                  d             i (p u1(x, t) + q u2(x, t))
           de1 := -- u1(x, t) = ---------------------------
                  dx                      lambda

> de2:=diff(u2(x,t),x)=(i/lambda)*(q*u1(x,t)-p*u2(x,t));

                  d             i (q u1(x, t) - p u2(x, t))
           de2 := -- u2(x, t) = ---------------------------
                  dx                      lambda

> de3 := diff(u1(x,t),t) = -(i*lambda/2)*u1(x,t)-q*u2(x,t);

             d
      de3 := -- u1(x, t) = - 1/2 i lambda u1(x, t) - q u2(x, t)
             dt

> de4 := diff(u2(x,t),t) = 1/2*i*lambda*u2(x,t)+q*u1(x,t);

              d
       de4 := -- u2(x, t) = 1/2 i lambda u2(x, t) + q u1(x, t)
              dt

> dsolve({de1,de2,de3,de4},{u1(x,t),u2(x,t)});