rt 错误：

terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid

代码如下

#include<iostream>
using namespace std;
int main(){
	string a=0;
	cin>>a;
	int ans=0;
	int len=0;
	len=a.length();
	for(int i=0;i<len;i++){
		if(a[i]=='1'){
			ans++;	
		}
	}
	cout<<ans;
	return 0;
}

提前谢谢大佬