Invalid operands to binary expression [xcode c++]

Why do I have this error in my code?

void check_letter(string letter, string entry) {
    for(int i = 0; i < entry.length(); i++) {
        if(entry[i] == letter) {

        }
    }
}

Invalid operands to binary expression ('std::basic_string<char>::value_type' (aka 'char') and 'std::string' (aka 'basic_string<char, char_traits<char>, allocator<char>>'))`

I want to check each letter in the word, so I check all letters of the word in a loop. I tried it on a array char (I made the array with individual letters from the word) and I had the same error.