how to handle this exception in c#..

hi..
i was converting xml value to pdf..

while converting this i got this exception"Index and length must refer to a location within the string. Parameter name: length"..

this is my code:
private static Bitmap returnBitMap(string sPixelData, Bitmap oBitmap, int iWidth, int iHeight)
{
try
{
iWidth=400;
iHeight=700;
string sCurrentPixelColor = String.Empty;

if (sPixelData != String.Empty)
{
oBitmap = new Bitmap(iWidth, iHeight);


int i = 0;
for (int ix = 0; ix < iHeight; ix++)
{

for (int iy = 0; iy < iWidth; iy++)
{
sCurrentPixelColor = sPixelData.Substring(i * 6, 6);
oBitmap.SetPixel(iy, ix, ColorTranslator.FromHtml("0x" + sCurrentPixelColor));
i++;
}


}


}
return oBitmap;
}
catch (Exception ex)
{

throw ex;
}

}


-----------------

how to handle this..

need ur suggestions..

regards
gopal.s