Using matlabFunction with a variable of two functions

Why is it, that two functions in a differential eq is not allowed with matlabFunction?

syms u(t) o(t)
V = u - o;
M = matlabFunction(V,'vars', {'t','Y'})

Gives the following warning "Warning: Function 'o' not verified to be a valid MATLAB function. ".

Seems like the first function u(t) is verified, but 'o' is not?