为什么语句1正常而语句2就会报错？

Question

StringBuilder sb=new StringBuilder();
StringBuilder sd=new StringBuilder();
for (int i = 0; i < 2; i++) {
    int index1=rd.nextInt(arr1.length);
    int index2=rd.nextInt(arr2.length);
    sb.append(arr1[index1]);
    sd.append(arr2[index2]);
}
System.out.println("验证码为："+sb+sd);//1
System.out.println(sb+sd);//2