Polymorphism and reference class member - wrong virtual method called

I was trying to understand how to apply polymorphism in C++ by creating a base class Document from which the two classes Book and Newspaper are derived. Note how the virtual method get_content() is overridden inside the derived classes.

class Document {
public:
  virtual std::string get_content() const { return ""; }
};

class Book : public Document {
public:
  std::string get_content() const override { return "Book"; }
};

class Newspaper: public Document {
public:
  std::string get_content() const override { return "Newspaper"; }
};

The Document class is aggregated with the Printer class (by reference), and the get_content() method of the appropriate derived class is called according to the type of document.

class Printer {
public:
  const Document& m_doc;

  Printer(const Document& doc): m_doc(doc) {
    std::cout << "Printing... " << m_doc.get_content() << std::endl;
  }

  std::string get_content() const { return m_doc.get_content(); }
};

Until now everything works well, but once I aggregate the Printer class with another class Binding, the get_content() doesn't seem to call the method of the right derived class anymore.

class Binding {
public:
  Printer m_printer;

  Binding(const Printer& printer): m_printer(printer) {
    std::cout << "Binding... " << m_printer.get_content() << std::endl;
  }
};

I don't understand why the following piece of code...:

int main() {
  Printer p1(Book{});
  Printer p2(Newspaper{});
  Binding b1(p1);
  Binding b2(p2);
}

...shows Binding... Newspaper at the third line while the Printing is clearly getting a Book instance as an input:

Printing... Book
Printing... Newspaper
Binding... Newspaper
Binding... Newspaper

Having m_printer declared as a reference inside Binding class doesn't fix this problem either.