How to convert a string to a type WWW

So I am stuck at the moment. Need to be able to get textures from online, but I want the player to be able to choose the texture. It is working fine when I put the url into the code, but when I try to get a value from an InputField it gives me an error.

The code that works:

 WWW www = new WWW("http://www.textures.com/system/categories/65/frontend-large.jpg");
 yield return www;
 texture = www.texture;

The code that doesn't work:

 WWW www = new WWW(textureInputField.text);
 yield return www;
 texture = www.texture;

It just keeps telling me the same thing... that my string doesn't match the type of variable it was looking for.

(or more explicitly)

"NullReferenceException: Object reference not set to an instance of an object UIScript.ApplyTexture () (at Assets/UIScript.cs:63)"

I know what the problem is, but I was wondering if there was a solution that I am oblivious to at the moment.