Lvalue to rvalue reference binding

The compiler keeps complaining I'm trying to bind an lvalue to an rvalue reference, but I cannot see how. I'm new to C++11, move semantics, etc., so please bear with me.

I have this function:

template <typename Key, typename Value, typename HashFunction, typename Equals>
Value& FastHash<Key, Value, HashFunction, Equals>::operator[](Key&& key)
{
    //  Some code here...

    Insert(key, Value()); // Compiler error here

    //   More code here.
}

which calls this method:

template <typename Key, typename Value, typename HashFunction, typename Equals>
void FastHash<Key, Value, HashFunction, Equals>::Insert(Key&& key, Value&& value)
{
    // ...
}

I keep getting errors like the following:

cannot convert argument 1 from 'std::string' to 'std::string &&'

on the Insert() call. Isn't key defined as an rvalue in the operator overload? Why is it being reinterpreted as an lvalue?