You appear to be trying to use the old calling sequence for sort. For example, from the man page of Solaris 8 sort:
Specify a field within each line to use as a sorting
And you may have been counting from the first field, which, in the old calling sequence, was "0", so you wanted to sort beginning with the sixth field.
However, you will likely need to use the new calling sequence:
start a key at POS1, end it at POS 2 (origin 1)
See man sort
for details -- it's not much of a change, you just need to get used to it ... cheers, makyo