MPI_Allgather generates Sigbus - fortran

Question

I'm new to fortran and MPI and currently processing a very large matrix row by row on different processors. I gather all the results on all the processors as it is needed. The following is the sample code which has the same structure as my real code.
I keep running into SIGBUS problems at MPI_Allgather, Line 49 of mod_test.f in the bounded function iter. How could I resolve this?
Compiler details:
$ mpifort --version
ifort (IFORT) 19.0.1.144 20181018
Copyright (C) 1985-2018 Intel Corporation. All rights reserved.
The code is compiled as follows:
mpifort mod_test.f main.f -o main -traceback -g -debug
mod_test.f
module TEST
include "mpif.h"
type A
real ,allocatable:: pf(:,:)
integer :: nx=100, ny=10
contains
procedure :: init
procedure :: iter
end type A
type(A) :: A_obj
contains
integer function init(this, x, y)
implicit none
class(A) , intent(inout):: this
integer , intent(in):: x, y
this% nx = x
this% ny = y
allocate( this% pf(x, y) )
this% pf = 0.0
init = 1
return
end function init
integer function iter(this, y_iter)
implicit none
class(A) , intent(inout):: this
integer , intent(in):: y_iter
integer :: i
real ,target :: a(this%nx+1), ar(this%nx+1)
real , dimension(:), pointer :: abuff, arbuff
a = 0.0
ar = 0.0
do i = 1, this% nx
this%pf(i, y_iter) = i * y_iter
enddo
a(1:this%nx) = this% pf(:, y_iter)
a(this%nx+1) = y_iter
call MPI_Allgather(a, this%nx+1, MPI_REAL, ar,
& this%nx+1, MPI_REAL,
& MPI_COMM_WORLD)
write(*,*) "Reached after MPI_Allgather"
do i = 1, this%nx + 1
write(*,*)ar(i)
enddo
this% pf(:, ar(this%nx+1)) = ar(1:this%nx)
write(*,*) "Got the solution from another processor"
iter = 1
end function iter
subroutine INIT_A
integer :: j, rank, ierr, size
! - Allocate
ierr= A_obj% init(100, 10)
! - Iterate
call MPI_COMM_RANK( MPI_COMM_WORLD, rank, ierr)
call MPI_COMM_SIZE( MPI_COMM_WORLD, size, ierr)
do j = 1, A_obj % ny
if ( rank == mod(j, size) ) then
ierr = A_obj % iter( j )
endif
enddo
end subroutine INIT_A
end module TEST
main.f
PROGRAM MAIN
use TEST
implicit none
integer :: ierr
call MPI_INIT(ierr)
call INIT_A
write(*,*) "Done with test"
call MPI_FINALIZE(ierr)
end PROGRAM MAIN