C#:添加两个一维数组的方法错误“；并不是所有的代码路径都返回一个值“；

我正在添加两个一维数组，但我在编写的代码中遇到了一个错误：错误：代码中存在未处理的异常。

static void Main()
{
 // setup two test arrays
 int[] x = new int[] { 2, 4, 6 };
 int[] y = new int[] { 3, 6, 9 };
 // invoke method and store result
int[] z = AddVectors(x, y);
}
static int[] AddVectors(int[] a, int[] b)
{
 // check that both arrays are of the same length
 if(a.Length == b.Length ) return null;
 // create a new array to store result
 int[] c = new int[a.Length];
 // carry out addition term by term
 for (int i = 0; i <= c.Length; i++)
 {
     c[i] = a[i] + b[i];
 }
 // return resulting array
  return c;
 }
}

if(a.Length != b.Length ) return null;