C# XNA error box "NullReferenceException was unhandled", "Object reference not set to an instance of an object."

Question

The exactly same line of code is used couple of lines below and it works just fine. My teacher couldn't figure it out. I use XNA in visual basic C#. When I run my game error message "NullReferenceException was unhandled", "Object reference not set to an instance of an object." is displayed.

Line that trows the error:

Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale),(int)(mBallTexture.Height * Scale));

My code:

 class RandomBall
        {
            //The asset name for the Sprite's Texture
            public string AssetName;

            //The size of the Sprite
            public Rectangle Size;

            //Used to size the Sprite up or down from the original image
            public float mScale = 1.0f;

            //The current position of the Sprite
            public Vector2 Position = new Vector2(500, 0);

            //The texture object used when drawing the sprite
            private Texture2D mBallTexture ;




            //When the scale is modified throught he property, the Size of the 
            //sprite is recalculated with the new scale applied.
            public float Scale
            {
                get {return mScale;}
                set
                {
                    mScale = value;
                    //Recalculate the Size of the Sprite with the new scale


    //THIS CODE LINE IS THE ONE CAUSING TROUBLE
    Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale), (int)(mBallTexture.Height * Scale));

                    ////Size = new Rectangle(0, 0, 50, 50);
                }
            }


            //Load the texture for the sprite using the Content Pipeline
            public void LoadContent(ContentManager theContentManager, string theAssetName)
            {
                mBallTexture = theContentManager.Load<Texture2D>(theAssetName);
                AssetName = theAssetName;
                Size = new Rectangle(0, 0, (int)(mBallTexture.Width * Scale), (int)(mBallTexture.Height * Scale));
            }