problem about comsol can't plot an external Matlab function that Nested a equations

Hi, everyone：

I have a problem about using an external matlab function that it nested another matlab function but comsol can't plot it.
this is my function:
>> function out = rho_x(T)
out=XSteam('rhoL_T',T);
%%XSteam is another M-File that contain a series of Material properties
This function can work in matlab,but not in comsol .
I have created another example to make out what is going on . That is :
>> function out = Cp_x(T)
out=CpL_x(T);
here is a M-File(CpL_x)
>>function out = CpL_x(T)
out=2+T.^2;
and is work ,that confused me a lot . Maybe something wrong about "XSteam('rhoL_T',T)", the 'rhoL_T' is just an char ,no an
numerical value.
Anyone with experience successfully doing this?

Thanks!
babyling