passing a map to a lambda function by value [duplicate]

This question already has an answer here:

• Why does C++0x's lambda require “mutable” keyword for capture-by-value, by default? 8 answers

Consider the following code:

int main() {
    list<string> names = { "ben", "ron", "dan" };
    char c = 'n';
    map<string, int> filtered;

    for_each(names.begin(), names.end(),
        [&filtered, c](const string& s) {
        if (s.find(c) != string::npos) { filtered[s]++; }
    });

    cout << "size is: " << filtered.size() << endl;
    for (auto& nameCountPair : filtered) {
        const string name = nameCountPair.first;
        cout << name << endl;
    }
}

When I remove the & inside the capture (i.e. [filtered, c]) I get the error:

no operator "[]" matches these operands

Why?