Why would indexing a map by tuple cause the "ambiguous" error in C++ in Xcode?

Question

I'm just gonna describe how to reproduce this problem:

I needed a map whose key is a tuple, and value is an integer. So I wrote this:

map<tuple<string, int>, int> someMap;

And I added one tuple to this someMap:

tuple<string, int> aTestTuple = make_tuple("whatever", 0);
someMap.insert(make_pair(aTestTuple, 2));

Everything so far was fine, until I wanted to get some value by key:

int test = someMap[aTestTuple];

The compiler in Xcode (5.0.2) complained:

/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib/c++/v1/map:1276:29: 
Call to member function '__construct_node' is ambiguous

Four candidates were found in the map header file:

//////////////////// CANDIDATE 1 ///////////////////////

#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
    __node_holder __construct_node();
    template <class _A0>
        typename enable_if
        <
            is_constructible<value_type, _A0>::value,
            __node_holder
        >::type
         __construct_node(_A0&& __a0);

//////////////////// CANDIDATE 2 ///////////////////////

    template <class _A0>
        typename enable_if
        <
            is_constructible<key_type, _A0>::value,
            __node_holder
        >::type
         __construct_node(_A0&& __a0);

//////////////////// CANDIDATE 3 ///////////////////////

#ifndef _LIBCPP_HAS_NO_VARIADICS
    template <class _A0, class _A1, class ..._Args>
        __node_holder __construct_node(_A0&& __a0, _A1&& __a1, _Args&& ...__args);
#endif  // _LIBCPP_HAS_NO_VARIADICS

//////////////////// CANDIDATE 4 ///////////////////////

#else  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
    __node_holder __construct_node(const key_type& __k);
#endif

    __node_base_pointer&
        __find_equal_key(__node_base_pointer& __parent, const key_type& __k);
    __node_base_pointer&
        __find_equal_key(const_iterator __hint,
                         __node_base_pointer& __parent, const key_type& __k);
    __node_base_const_pointer
        __find_equal_key(__node_base_const_pointer& __parent, const key_type& __k) const;

So the question is: What should I do if I want to index a map by tuples in C++?