G++ Compiler Error or faulty code? : "template definition of non-template"

As part of a traits class for a larger program, I attempted to create a static class variable which could have a different value depending on the type that the enclosing class template was instantiated with.

I've simplified the relevant code to produce a bare-bones example of what I'm talking about:

#include <iostream>
#include <string>
#include <type_traits>

template <class T, class Enable = void>
struct Foo;

template <class T>
struct Foo<T,
    typename std::enable_if<std::is_integral<T>::value>::type
>
{
    static std::string message;
};

template <class T>
struct Foo<T,
    typename std::enable_if<std::is_floating_point<T>::value>::type
>
{
    static std::string message;
};

template <class T, class Enable>
std::string Foo<T, Enable>::message;

With GCC 4.6, this gives a compiler error: template definition of non-template ‘std::string Foo<T, Enable>::message. The problem occurs because of the last two lines, where I'm just defining the static variable std::string Foo<T, Enable>::message.

I'm confused why this is happening. The compiler error goes away if I omit the last two lines (but of course that causes linker errors.) Is this a compiler error with GCC?