Non-member operators

I have declared a binary non-member operator+ in the below class.

class superBob {
public:
	superBob(int a) : n(a) {}
	
	int getInt() const {
		return n;
	}
	friend int operator+(int,int);
	
protected:
	int n;
};


I have read that non-member binary operators takes two arguments, but 
when compiling I get:

‘int operator+(int, int)’ must have an argument of class or enumerated
type


As the error indicates one of the arguments must be of class or 
enumerated type. Redefining to:

friend int operator+(const superBob&, int);

compiles without error. But why does one of the arguments have to be of 
class or enumerated type?