Why does my program causes a segmentation fault?

I wrote a very basic program in Fortran using one function written by my teacher:

implicit none
integer, parameter :: entero=SELECTED_INT_KIND(9)
integer, parameter :: doblep=SELECTED_REAL_KIND(15,307)
integer (kind=entero), parameter :: r=3
real (kind=doblep) , external:: mi_random
real (kind=doblep) :: m

m=mi_random(r)
write (*,*) mi_random(r)

STOP
END PROGRAM random_num

Basically, it prints the result of the function. I'm getting a segmentation error everytime the function is ran, though.

Here is the function written by my teacher, but this part should be okay, the issue should be on the program above. It is a function that generates a random number between 0 and 1:

      function mi_random(idum)

      integer, parameter :: entero=SELECTED_INT_KIND(9)
      integer, parameter :: doblep=SELECTED_REAL_KIND(15,307)

      implicit none
        
      integer (kind=entero) :: idum
      real (kind=doblep), parameter :: mbig=4.d+06,mseed=1618033.d00
      real (kind=doblep), parameter :: mz=0.d00,fac=1.d00/mbig
      integer (kind=entero) :: i,iff,ii,inext,inextp,k
      real (kind=doblep) :: mi_random,mj,mk,ma(55)
      save iff,inext,inextp,ma
      data iff/0/

      if (idum<=0.or.iff==0) then
            iff=1
            mj=abs(mseed-abs(idum))
            mj=mod(mj,mbig)
            ma(55)=mj
            mk=1
            do i=1,54
                  ii=mod(21*i,55)
                  ma(ii)=mk
                  mk=mj-mk
                  if (mk.lt.mz) mk=mk+mbig
                  mj=ma(ii)
            end do
            do k=1,4
                  do i=1,55
                        ma(i)=ma(i)-ma(1+mod(i+30,55))
                        if (ma(i).lt.mz) ma(i)=ma(i)+mbig
                  end do
            end do
            inext=0
            inextp=31
            idum=1
      end if
      inext=inext+1
      if (inext==56) inext=1
      inextp=inextp+1
      if (inextp==56) inextp=1
      mj=ma(inext)-ma(inextp)
      if(mj.lt.mz) mj=mj+mbig
      ma(inext)=mj
      mi_random=mj*fac
      return
      end function mi_random```