Clang static analysis doesn't like default parameter for const string&

Question

I am getting the following error with clang static analyzer. Any ideas?

error: reference to type 'const std::string' (aka 'const basic_string<char>') could not bind to an lvalue of type 'const char [1]'
74
    int foo(const std::string& data, const std::string& key = "");

header:

class Test
{
public:
    Test();
    int foo(const std::string& data, const std::string& key = "");
};

src:

int Test::foo(const std::string& data, const std::string& key)
{
  //stuff
}