Simple scrolling background (2D). A small problem with my oneline code

Why whont this code work properly?

public float speed = 0.2; void Update(){ renderer.material.mainTextureOffset = new Vector2(Time.time * speed, 0f); }

So it will scroll, but not start over from the beginning. Insted the image is copying itself at the end, making a mess.

Where is my mistake? I have used it before, and it worked then. Im really confused