How to declare a constant variable in Data Store Memory ?

Hello,

In a Matlab Function Bloc, I initialize the output with the zero() function and a global variable :

global var
Output = zeros(var,1);

With var declared in a Data Store Memory bloc. My problem is that Output is inferred as a variable size vector, which it is not because var is a design parameter and thus is constant during simulation. Is there a way to specify Simulink that var doesn't change ?