Why is this user-defined conversion failing?

This code fails but I think the conversion function chosen should be operator std::string() since it takes less steps (Foo -> std::string) than the operator const char*() (Foo -> const char* -> std::string).

struct Foo
{
    operator const char*();
    operator std::string();
};

void f(Foo foo)
{
    std::string s2(foo); // Error!
}

The standard says the conversions are limited to one conversion sequence, and that conversions are applied only where they are unambiguous, so why are these failing?

The error is:

error: call to constructor of 'std::string' (aka 'basic_string<char>') is ambiguous
    std::string s2(foo);
                ^  ~~~
note: candidate constructor
      basic_string(basic_string&& __str)
      ^
note: candidate constructor
      basic_string(const basic_string& __str);
      ^
note: candidate constructor
      basic_string(const _CharT* __s, const _Alloc& __a = _Alloc());
      ^
1 error generated.