'System.ArgumentOutOfRangeException' occurred in mscorlib.dll

'System.ArgumentOutOfRangeException' occurred in mscorlib.dll

Thank you in advance! I'm writing an application to read a txt file and
write to Excel. To start small I'm just trying to read the first line.
The Console.WriteLine correctly output the string (which has blank spaces
between characters), but when I try to extract the characters from the
string I get this error: Additional information: Index and length must
refer to a location within the string.
while ((input = stream.ReadLine()) != null)
{
//ACH HEADER LINE
if (iCurRecLine == 0 && input.IndexOf(HeaderBeginKeyWord) >= 0)
{
Console.WriteLine(input);
RepDate = input.Substring(23, 6).Trim();
RecordTypeCode = input.Substring(0, 1).Trim();
PriorityCode = input.Substring(1, 2).Trim();
ImmDestination = input.Substring(3, 10).Trim();
ImmOrigin = input.Substring(13, 10).Trim();
FileCreatedDate = input.Substring(23, 6).Trim();
FileCreatedTime = input.Substring(29, 4).Trim();
FileIDModifier = input.Substring(33, 1).Trim();
RecordSize = input.Substring(34, 3).Trim();
BlockingFactor = input.Substring(37, 2).Trim();
FormatCode = input.Substring(39, 1).Trim();
Destination = input.Substring(40, 23).Trim();
Origin = input.Substring(63, 23).Trim();
ReferenceCode = input.Substring(86, 8).Trim();
...
}
}
Where did I go wrong on this?