Stoi raises out of range error

I have this code which is giving me error terminating with uncaught exception of type std::out_of_range: stoi: out of range

which I identified as being caused by the line long ascii = std::stoi(temp_string);

what about how i am using stoi

causes this and how to fix it?

std::string encode(std::string message){
std::string num_value;
long cipher;
if(message.length() < 20){
  for(int i = 0; i < message.length(); ++i){
    int temp = (char) message.at(i); 
    num_value += std::to_string(temp); 
  }
  return num_value;
}
else{
    int count = 0;   
    std::string temp_string;
    for(int i = 0; i < message.length(); ++i){
      int temp = (int)  message.at(i);
      temp_string += std::to_string(temp);           
      count++;   
       if(count == 20){
         count = 0;
         //add cipher encrypt
         long ascii = std::stoi(temp_string);
         //cipher = pow(ascii, 10000);
         //add n to cipther encrypt
         //add cipherencrypt + n to num_value
         //reset temp_string
         temp_string += "n";

         temp_string = ""; 

      }
 }
return num_value;
}

int main(){
  std::string s = "Hello World my t your name aaaaaaaaaaaaa?";
  std::cout<<"encoded value : "<< encode(s)<<std::endl;
}