Ошибка при переопределении setter

Ошибка "Несогласованность по доступности" на "SpeedVector" у "public Vector SpeedVector".

struct Vector
{
    public float x;
    public float y;
    public Vector(float x, float y)
    {
        this.x = x;
        this.y = y;
    }
    public static Vector operator +(Vector v1, Vector v2)
    {
        return new Vector(v1.x + v2.x, v1.y + v2.y);
    }
}
public class GameObject : Control
{
    public float mass;
    private Vector speedVector = new Vector(0, 0);
    public Vector SpeedVector
    {
        set
        {
            if (mass != 0)
            {
                speedVector = value;
            }
        }
        get => speedVector;
    }
    public void ElliminateCollision(GameObject obj1, GameObject obj2)
    {
    }
}