FluidProp ERROR : No method 'Mmol' with matching signature found for class 'COM.Fluid​Prop_Fluid​Prop'

Hi everybody, I'm trying to call FluidProp function from Matlab environment. I initialize the COM server as follow:

FP = actxserver ('FluidProp.FluidProp')

When I try to call functions directly, all works great; for instance, the following commands gives the molar mass of methane:

invoke(FP,'SetFluid_M','GasMix',1,'CH4',1);     
mm_ch4 = FP.Mmol;

The problem arise when I try to call RefProp functions into a linear algebraic equations system,

syms a b;
eqns = [ a == FP.Mmol, b == a + 2 ];
vars = [a b];
[A,b] = equationsToMatrix(eqns,vars);
S = double(linsolve(A,b));

which gives the error output:

No method 'Mmol' with matching signature found for class 'COM.FluidProp_FluidProp'

How can I solve the problem? Thanks in advance, MR

[Matteo Rocco]

I found that the fluidprop functions cannot work with symbolic variables. Maybe this is the root of the problem.

How can i solve it? Thanks