Function adding sums of 2 arrays not working, why?

// Jones, Bradley L.; Peter Aitken; Dean Miller. C Programming in One Hour a Day, Sams Teach Yourself (p. 357). Pearson Education. Kindle Edition.
// Write a function named sumarrays() that accepts two arrays as arguments, totals all values in both arrays, and returns the total to the calling program.

int array[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
int array2[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};

int sumarrays(int arrayc[], int arrayc2[]);

#include <stdio.h>

int main() {

   int sum = 0, sum2 = 0, loop;

   for(loop = 9; loop >= 0; loop--) {
      sum = sum + array[loop];
   }


   for(loop = 9; loop >= 0; loop--) {
      sum2 = sum2 + array2[loop];
   }

   printf("Sum of array is %d.\n", sum);
   printf("Sum of array2 is %d.\n", sum2);
   printf("Sum of array + sum of array2 is %d.\n", sum + sum2);
   printf("Sum of array + sum of array2 is %d (through function sumarrays()).", sumarrays(array, array2));

   return 0;
}

int sumarrays(int arrayc[], int arrayc2[])
{

int sum = 0, sum2 = 0, loop, loop2;

    for(loop = 9; arrayc[loop] >= 0; loop--) {
      sum = sum + arrayc[loop];
   }

    for(loop2 = 9; arrayc2[loop2] >= 0; loop2--) {
      sum2 = sum2 + arrayc2[loop2];
   }

    return sum + sum2;
}