existing records  in  relation  <str>  would  violate  VALID  IF
  expression

  Explanation:   When  a  new  field  is  added  to  a   relation,
  previously stored records will have the new field's value set to
  NULL (MISSING).  In  your  relation,  the  VALID  IF  constraint
  defined for the new field is violated by this action.

  User Action:  Modify all the records where the new field is  set
  to  NULL (MISSING) to have a non-null value, or change the VALID
  IF clause for the field to allow MISSING values.