System.ArgumentOutOfRangeException: Index and length must refer to a location within the string

{
{
additionalcharges =
}
if (transinfodum.additionalcharge.ToString() != "")if (transinfodum.additionalcharge.ToString().Substring(0, 1) == "-")"Discount " + transinfodum.additionalcharge.ToString().Substring(1);else{
additionalcharges = transinfodum.additionalcharge.ToString();
}
}
 

this is takeplace when i was getting a sub string from a null string value.
solved it...