Weird issue using static map in c++

I have an issue that I don't understand why it happens. In a nutshell, I would like to create a singleton to read a configuration file where each property will be stored in a map.

I defined my configuration class (.h) as

class Configuration {
public:
    static Configuration* getInstance(std::string configFile);

private:
    static std::map<std::string, std::string> properties;
    static bool instanceFlag;
    static Configuration *instance;

    Configuration();
    Configuration(std::string configFile);
    ~Configuration();
};

On the implementation class I have the following:

#include <iostream>
#include <fstream>

#include "Configuration.h"

std::map<std::string, std::string> Configuration::properties {};
bool Configuration::instanceFlag = false;
Configuration *Configuration::instance = NULL;

The issue is, my variable properties is the type of Configuration instead of a map. If a replace the key and value to be a int, the properties becomes a map. It seems weird.

Have you seen it before? What am I missing here?

Thanks Mauro