Problem with a boundary condition in Comsol

hi, i'm a chemical engineering student with a little problem with Comsol multiphysics;
in practice, i have to solve a problem of diffusion in a solid sphere.
after drawing the domain, i have to set a boundary condition on sphere's surfaces.
this condition, for my problem, is FLUX=Kc(Cb-C) and Cb (bulk concentration) is

proportional to an integration coupling variable, that i have previously fixed
In particular, Cb is equal to costant * mass_released, where mass_released is equal

to volume integral of C0 (initial concentration) minus C

After setting all variables and running simulation, it gives me an error referred to

evaluation of variable Cb
I think the problem is that it's difficult to use in a boundary condition a variable that

changes during simulation (in fact it is time dependent)..

I hope someone can help me, have a nice day!!!

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