Why doesn't this work (inheritance & namespaces)

[twanvl]

This code gives an error ("illegal member initialization: 'ifstream' is not a base or member")

#include <fstream>

namespace MyNamespace{
    class MyClass;
}

class MyNamespace::MyClass : public std::ifstream{
public:
    MyClass() : std::ifstream() {}
};

but this code does not:

#include <fstream>
using std::ifsream;

namespace MyNamespace{
    class MyClass;
}

class MyNamespace::MyClass : public ifstream{
public:
    MyClass() : ifstream() {}
};

What am I doing wrong?

EDIT: I'm using MSVC 6