not enough host variables passed; <num> passed, at  least  <num>
  used

  Explanation:   You  did  not  pass  enough  host  variables   to
  RDB$INTERPRET.   For RDB$INTERPRET, there were more !VALs in the
  command string than host variable descriptors  passed.   If  one
  more  host  variable  is detected by the parser than was passed,
  parsing stops.  There could be more host variables used than the
  error message specifies.

  User Action:  Supply  matching  number  of  host  variables  and
  !VALs.