Question: pdsolve - boundary condition

Hallo Everybody,

I hope someone can help me.

I want to solve numerically the diffusion equation,

          diff(csh(rh, t), t) = Dif*(diff(rh^2*(diff(csh(rh, t), rh)), rh))/(r0*rh^2)

          where "Dif" is the diffusivity and "r0" is the initial value of rh (the equations were normalized).

subject to the following boundary and initial conditions:

          IBC := {csh(rh, 0) = csh(rh[i-1], t1), (D[1](csh))(0, t) = 0, (D[2](csh))(rh[i], t) = -2*Dif*csh(rh[i], t)/(r0*rh[i])}

As the boundary is moving I made myself a discretization in time to take it in account. So the pde is solved i times, each interval with a new radius (rh).

When I try to solve this problem, Maple returns the following error message:

Error, (in pdsolve/numeric/process_IBCs) initial/boundary conditions can only contain derivatives which are normal to the boundary, got (D[2](csh))(1.000000000, t)

Someone knows what this error mean and how can it be solved?

Thank you in advance,

Luciane.