I am getting an error "Spatial discretization has failed" while solving a 1D pde using pdepe for spherical coordinates

I want to solve a pde for diffusion of species inside a spherical bacterial cell from its SURFACE.

The boundary conditions are as below: at r=0, dC/dr=0 for all t>0; and at r=R, C=1200 for all t>0; [--> **No convection from outside environment to the cell surface is involved, only diffusion is taking place from the cell surface to the inside of the cell]

I have written Boundary conditon as below for pdepe:

    function [pl,ql,pr,qr] = PDEBC(xl,ul,xr,ur,t)
    global K Ce De D
    pl = 0;
    ql = 1;
    pr = (ur-1200);
    qr = 0;

Following error occurs: Spatial discretization has failed

However, everything goes fine if I take convective and diffusive transport together into consideration, with BC: @ r=R, DdC/dr=K(C-1200)

I request the community to please help me.

Thanks in advance.