Why Can't I use a mem_fn Functor in bind?

I wanted to pass a mem_fn argument to bind but the compiler doesn't seem to allow it.

For example this works fine:

accumulate(cbegin(foos), cend(foos), 0, bind(plus<int>(), placeholders::_1, bind(&foo::r, placeholders::_2)));

But when I try to use the mem_fn functor I get about a page of errors:

accumulate(cbegin(foos), cend(foos), 0, bind(plus<int>(), placeholders::_1, mem_fn(&foo::r)));

/usr/include/c++/6/bits/stl_numeric.h: In instantiation of ‘_Tp std::accumulate(_InputIterator, _InputIterator, _Tp, _BinaryOperation) [with _InputIterator = __gnu_cxx::__normal_iterator >; _Tp = int; _BinaryOperation = std::_Bind(std::_Placeholder<1>, std::_Mem_fn)>]’:
prog.cpp:20:102: required from here
/usr/include/c++/6/bits/stl_numeric.h:154:22: error: no match for call to ‘(std::_Bind(std::_Placeholder<1>, std::_Mem_fn)>) (int&, foo* const&)’