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

Hi,
I have this string:

22/11/11 19:51     100 01 <I>399AB2C4               1'04 00:00'00            NA

and I am trying to read a part of using this code"
cdr_dial_number = cdr_raw.Substring(26, 15);

but getting this error:
Index and length must refer to a location within the string.
Parameter name: length why is that? ...