C++11: error when trying to return pair via async/future

Hoping that this is just a dumb mistake on my part. This is my first venture into multithreading, so I don't know what I'm doing.

Here is the code in question (on my red black tree):

pair<unsigned long long, unsigned long long> ALTNode::height()    {
    pair<unsigned long long, unsigned long long> hl(0,0), hr(0,0), f;
    future<pair<unsigned long long, unsigned long long>> l, r;
    if (left)   
        l = async(std::launch::async,left->height());
    if (right)
        r = async(std::launch::async,right->height());
    if (left)
        hl = l.get();
    if (right)
        hr = r.get();
    f.first = ((hl.first < hr.first) ? hl.first : hr.first) + 1;
    f.second = ((hl.second > hr.second) ? hl.second : hr.second) + 1;
    return f;
}

Here is the generated error:

Error   1   error C2664: 'std::pair<_Ty1,_Ty2>::pair(const std::pair<_Ty1,_Ty2> &)' : cannot convert parameter 1 from 'std::_Do_call_ret<_Forced,_Ret,_Funx,_Btuple,_Ftuple>::type' to 'const std::pair<_Ty1,_Ty2> &'   G:\Visual Studio 2012\VC\include\xrefwrap   431 1   ALT-Tree

Can you help me out here?