GetPictureUrl Unhandled Exception

In the PictureManager.cs class the public static string GetPictureUrl(Picture picture, int targetSize, bool showDefaultPicture) function throws an "Object reference not set to an instance of an object" error on the following line:
...
else
                {
                    localFilename = string.Format("{0}_{1}.{2}", picture.PictureId.ToString("0000000"), targetSize, lastPart);
                   if (!File.Exists(Path.Combine(PictureManager.LocalThumbImagePath, localFilename)))