only text and varying text fields may be shortened in indices

  Explanation:  You attempted to specify a shortened size  for  an
  index  field,  but  specified  a  data  type  other than text or
  varying text.  Only fields with a text or varying text data type
  can be shortened in an index.

  User Action:  Review the command you typed, and issue  it  again
  specifying a text or varying text field if appropriate.