Isend/Irecv doesn`t work but Send/Recv does

Isend/Irecv doesn`t work but Send/Recv does

When I use Send/Recv my code works but when I replace Send/Recv with
Isend/Irecv it yields segmentation fault. But before going anywhere else I
wanted to verify whether the following snippet seems alrite or not.
The rest of the code should be fine as Send/Recv works; but I have pasted
here as its long.
DO I=1,52
REQ(I)=MPI_REQUEST_NULL
ENDDO
IF (TASKID.NE.0) THEN
NT=TASKID
CALL
MPI_ISEND(RCC(IIST:IIEND,JJST:JJEND,KKST:KKEND),SIZE(RCC),MPI_DOUBLE_PRECISION,0,8,MPI_COMM_WORLD,REQ(NT),IERR)
ENDIF
IF (TASKID.EQ.0) THEN
DO NT = 1,26
CALL
MPI_IRECV(CC(RSPANX(NT):RSPANXE(NT),RSPANY(NT):RSPANYE(NT),RSPANZ(NT):RSPANZE(NT)),SIZECC(NT),MPI_DOUBLE_PRECISION,NT,8,MPI_COMM_WORLD,REQ(NT+26),IERR)
ENDDO
ENDIF
CALL MPI_WAITALL(52,REQ,ISTAT,IERR)