关于左值的错误

#include<iostream>
#include<string> 
using namespace std;

class TextBlock{
public:
    TextBlock(string x):text(x){
    }
    //operator[] for const 对象
    const char& operator[](std::size_t position) const
    { return text[position]; }

    //operator[] for non-const 对象
    char operator[](std::size_t position)
    { return text[position]; }
    
private:
    std::string text;
};

int main(){
    TextBlock tb("hello");
    tb[0] = 'x';      //报错： lvalue required as left operand of assignment
        
    std::cout << tb[0] << endl;
    
    
    const TextBlock ctb("World!!");
    std::cout << ctb[0]; 
    
}