std::reference_wrapper on MS Visual Studio 2013

I try to compile some code which is very similar to:

#include <string>
#include <unordered_map>

class A{
};

int main(int argc, char* argv[]){
  std::unordered_map<std::string, std::reference_wrapper<const A>> stringToRef;
  A a;
  const A& b = a;
  stringToRef.insert(std::make_pair("Test", b));
  return 0;
}

But can't figure out, why it's not compiling. I'm pretty sure, that the same code compiled fine on MS Visual Studio 2012 - but on Visual Studio 2013, it reports the following compilation error:

error C2280: std::reference_wrapper<const A>::reference_wrapper(_Ty &&): attempting to reference a deleted function

I tried to add copy, move, assignment operators to my class - but couldn't get rid of this error. How can I find out exactly, which deleted function this error refers to?