Cast Object - get null

[Namespace]

override bool opEquals(Object o) const {
        if (o is null) {
                return false;
        }
        
        writeln(o); // write: cast.Vector2D!(float).Vector2D
        
        Vector2D!(T) vec = cast(Vector2D!(T)) o;
        
        writeln(vec); // write: null
        // ...

It seems the cast fail, but the question is: why?

Here my test context:

void main() {
        alias Vector2D!(short) Vector2s;
        alias Vector2D!(float) Vector2f;
        
        Vector2f vf = Vector2f(23, 42);
        
        Vector2s vs = Vector2s(vf);
        
        writefln("vs.x: %d, vs.y: %d", vs.x, vs.y);
        
        Vector2s vs2 = Vector2s(23, 42);
        
        if (vs2 == vf) {
                writeln("equal");
        }
}

Vector2D is my own class. If i compare vs2 with vs it works fine,
but if i compare vs or vs2 with vf, the cast fail and i get a
null-reference. How can i avoid this?