Problem passing structure with array into function

The structure:

typedef struct{
     int val[256];
}values;

The function(needs to pass information into the structure array 'val'):

void changeval(values *obj, int v, int a){
     (*obj).val[v]=a;
}

^^That piece is the piece I am having issues with...

And finally, the call:

int main(){
     values item;
     changeval(&item,0,50);
     return 0;
}

Ok, why can't I pass a user-defined datatype into a function and be able to access the array that is in the object? :sad: