How can I fix this error

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C++

Linux

#include <stdio.h>
#include <mpi.h>
#include 
#include <time.h>
#include <math.h>
#define size 1000
float a[size][size];
float sum[size];

void norm(int istart, int iend)
{
int i,j;
    for ( i = istart; i <= iend; ++i) {
        for (j = 0; j < size; ++j) {
           sum[i]=sum[i]+a[i][j];
        }
    }
}

int main(int argc, char* argv[])
{
    int rank, nproc,i,j;
    int istart, iend;
float t_1;
clock_t c_1,c_2;
c_1=time(NULL); // time measure: start mm
    MPI_Init(&argc, &argv);
    MPI_Comm_size(MPI_COMM_WORLD, &nproc);
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    if (rank == 0) {
        // Initialize buffers.
        for ( i = 0; i < size; ++i) {
            for (j = 0; j < size; ++j) {
                a[i][j] = (float)i + j;
            }
 sum[i]=0;
        }
    }
    MPI_Bcast(a, size*size, MPI_FLOAT, 0,MPI_COMM_WORLD);
    istart = (size / nproc) * rank;
    iend = (size / nproc) * (rank + 1) - 1;
    norm(istart, iend);
    MPI_Gather(sum+ (size/nproc*rank),
               size*size/nproc,
               MPI_FLOAT,
               sum + (size/nproc*rank),
               size*size/nproc,
               MPI_FLOAT,
               0,
               MPI_COMM_WORLD);

    if (rank == 0) {
        if (size % nproc > 0) {
            norm((size/nproc)*nproc, size-1);
        }
    }
    MPI_Finalize();
c_2=time(NULL); 
t_1 = (float)(c_2-c_1); 
printf("Execution time: %f \n",t_1);
    return 0;
}

What I have tried:

Fatal error in PMPI_Gather: Invalid buffer pointer, error stack:
PMPI_Gather(856): MPI_Gather(sbuf=0x6010a0, scount=1000000, MPI_FLOAT, rbuf=0x6010a0, rcount=1000000, MPI_FLOAT, root=0, MPI_COMM_WORLD) failed
PMPI_Gather(797): Buffers must not be aliased

it program to compute norm1 and infinite norm in mathmatical
it run ....
but when it get run show me this error
what means of this error
please help