Crash when inserting element into std::map

`The following function compiles using Xcode 6.1:

void attach(const std::string& command, std::function<void()> callback)
{
    std::vector<std::function<void()>> v;
    v.push_back(callback);
    m_observers.insert(std::make_pair(command, v));   // causes the crash
}

but when it is called as follows:

attach("xxx", std::bind(&X::callback, this));

it crashes in a function called __tree_balance_after_insert in __tree in the std library.

m_observers is declared as:

std::map<std::string, std::vector<std::function<void()>>> m_observers;

X::callback is declared as void X::callback().

Can someone explain what is going wrong?