Segmentation fault when filling a matrix occurs at the first index

I'm working on this C program and I've run into an issue with filling a C matrix. Whenever I try to fill the matrix in my update function, I'm hit with a segmentation fault. I don't know if it's an issue with how I'm passing, initializing, or modifying the array.

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>

int n,m, i, j;
double count = 0;

void update(int n, int m, double **arr, double *arr1){
  printf("called update \n");
  for(i=0; i<n; i++){
    for(j=0; j<m; j++){
      count++;
      arr[i][j] = count;
      arr1[i] = arr[i][j];
    }
  }
}


int main(int argc, char * argv[]) {
  assert(argc==3);
  n = atoi(argv[2]);
  m = atoi(argv[1]);
  double *d_a;

  double**arr=(double**)malloc(n*sizeof*arr);
  double*arr1=(double*)malloc((n*m)*sizeof*arr1);
  double*sum_d1=(double*)malloc(m*sizeof*sum_d1);

  for(int i=0; i<n;i++){
    arr[i] = (double*)malloc(m*sizeof*(arr[i]));
    for(int j=0; j<m; j++){
      arr[i][j] = i*m+j;
    }
  }

  for(int i=0; i<n; i++){
    for(int j = 0; j<m; j++){
      arr1[i*m+j]=arr[i][j];
    }
  }

  update(n,m,arr, arr1);
  return 0;
}