Linear congruential generator in Fortran 77 - output is all 0s?

I've been trying to make a pretty basic LCG pseudorandom number generator in Fortran 77 to print out 1000 random numbers to a file, but for whatever reason the output is just 1000 0s. The entire code is pretty short so I've combed it over multiple times and tried changing some things around but I can't for the life of me figure out what is wrong. I have a hunch that it could be a scope issue (if such a concept is even useful in Fortran), but that is really unfounded.

      PROGRAM RANDOM
      COMMON ISEED, RANDOMNUMBER
      ISEED = 123
      OPEN (UNIT=1,FILE='rand.in',STATUS='UNKNOWN')

      J=1

    7 CALL RANDU(ISEED)
      J=J+1
      WRITE(1,*) RANDOMNUMBER
      IF(J<1000)GOTO 7

      STOP
      END

      SUBROUTINE RANDU(ISEED)
      PARAMETER (IMAX = 2147483647, IMAXINV = 1./IMAX)
      ISEED = ISEED * 65539
      IF(ISEED<0) ISEED = ISEED + IMAX + 1
      RANDOMNUMBER = ISEED * IMAXINV
      RETURN
      END

Does anyone have any ideas here? I'm fresh out.