error: invalid initialization of reference of type 'int&' from expression of type 'const int'

This is an unrelated question about the code in this question, regarding the following template function.

template <class T>
class Object : public Container {
public:
    T& object;

    Object(const T& obj) : object(obj) {}
};

This is the code that calls the constructor:

template <class T>
void Array::add_element(const T& element)
{
    vec.push_back(new Object<T>(element));
}

This code compiles fine, but as soon as I add a line in main that calls it:

Array array;
int i = 3;
array.add_element(i);

I get a compiler warning: error: invalid initialization of reference of type 'int&' from expression of type 'const int'.

What is that about? I passed an int in. Shouldn't it automatically be turned into a const int& for me? Why is the compiler complaining?