aupper.for(12): error #8000:  There is a conflict between local interface block and external interface block.   [FYUPPR]

Code:

      program aupper
      IMPLICIT NONE
      INTERFACE
        FUNCTION FYUPPR(STRING) RESULT(FYUPPR_0)
          CHARACTER(*) :: STRING
          CHARACTER(LEN=LEN(STRING)) :: FYUPPR_0
        END FUNCTION FYUPPR
      END INTERFACE
      CHARACTER(256) :: carg

      call getarg(1,carg)
      carg = fyuppr(carg)
      END program aupper

      FUNCTION FYuppr (STRING)
      IMPLICIT NONE
      CHARACTER(LEN=*) STRING
      CHARACTER(LEN=LEN(STRING)) FYuppr
      FYuppr = STRING
      RETURN
      END FUNCTION FYuppr