Question: invalid subsricpt selector

tryint to solve a series of d.e.

> restart;
> assume(1 < Ha);
> Ha := 30;
                                     30
> eq1 := Ha*(diff(b(y), y))+diff(u(y), `$`(y, 2)) = -1;
                   
> eq2 := Ha*(diff(u(y), y))+diff(b(y), `$`(y, 2)) = 0;
                     
> BC1 := eval(u(y), y = -1) = 0;
                                  u(-1) = 0
> BC2 := eval(u(y), y = 1) = 0;
                                  u(1) = 0
> BC3 := (D[1](b))(-1) = 0;
                                D(b)(-1) = 0
> BC4 := (D[1](b))(1) = 0;
                                 D(b)(1) = 0
> pds := pdsolve([eq1, eq2], [BC1, BC2, BC3, BC4], numeric, [u(y), b(y)], spacestep = 0.5e-1);
Error, (in pdsolve/numeric/process_PDEs) invalid subscript selector
>
 

WHY IS THIS HAPPENING TO ME?