Why doesn't g++ recognize stoi even using C++11?

I have this function in C++11:

bool ccc(const string cc) {
    
vector<string> digits;
    
int aux;
    
for(int n = 0; n < cc.length(); ++n) {
    
digits.push_back(to_string(cc[n])); }
    
for(int s = 1; s < digits.size(); s += 2) {
    
aux = stoi(digits[s]);
    
aux *= 2;
    
digits[s] = to_string(aux);
    
aux = 0;
    
for(int f = 0; f < digits[s].length(); ++f) {
    
aux += stoi(digits[s][f]); }
    
digits[s] = to_string(aux);
    
aux = 0; }
    
for(int b = 0; b < digits.size(); ++b) {
    
aux += stoi(digits[b]); }
    
aux *= 9;
    
aux %= 10;
    
return (aux == 0); }

And I get this error when compiling with g++ with the -std=c++11 flag:

crecarche.cpp: In function ‘bool ccc(std::string)’:
    
crecarche.cpp:18:12: error: no matching function for call to ‘stoi(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)’
    
18 | aux += stoi(digits[s][f]); }
        |        ~~~~^~~~~~~~~~~~~~

But I used the stoi function after and I did not get any error with that line.

Why is the compiler throwing me this error and how can I fix it?