reference to  segmented  string  <str>  must  use  RDB$VALUE  or
  RDB$LENGTH

  Explanation:  You attempted to use a context variable  that  was
  defined  in  a  segmented  string RSE with a field name that was
  neither RDB$VALUE or RDB$LENGTH, or you  attempted  to  ERASE  a
  segmented  string  segment using that context variable.  You can
  only use RDB$VALUE or RDB$LENGTH to refer to a segmented  string
  contents  and individual segments can not be erased using RDO or
  RDBPRE.

  User  Action:   Use   either   <context-variable>.RDB$VALUE   or
  <context-variable>.RDB$LENGTH, where context variable is the one
  defined in  the  segmented  string  RSE.   Or  erase  an  entire
  segmented string by assigning zero to the segmented string field
  name.