MATLAB PLOT of composed function

I am trying to plot a composed function as long I would like to design an electric cam. The code is very simple:

% Function Composition

%t = 0:0.1:3;
syms x t
g = sawtooth(t);
f = x;

a = compose(f,g)
fplot(a)

But it returns me:

>> Function_composition Error using Function_composition (line 8) Input arguments must be 'double'.

I really need to be able to do so.

Any hints?