Coefficient Handle for Nonconstant Coefficients

In specifyCoefficients there is an option to choose the coefficient "f" as a non-constant, for instance in the following form:

f = @(location,state)location.y.^2.*tanh(location.z)/1000;

I need to set f as follows:

a=@(t,z) interp2(XX,TT,V,t,z);

f = @(location,state) a(location.y,location.x);

This gives the following error:

Error using pde.PDEModel/solveStationaryNonlinear (line 27)

Unsuitable initial guess U0 (default: U0=0).

Error in pde.PDEModel/solvepde (line 77)