The resistance (R) of an object is defined as the ratio of voltage across it (V) to current through it (I), so why can't I claim that the internal resistance of a voltage source is: r=V/I? (V is the voltage across the voltage source and I is the current that flow through the voltage source)

The resistance (R) of an object is defined as the ratio of the voltage across it (V) to the current that flows through it (I), so why can't I claim that the internal resistance of a voltage source is: r=V/I? (V is the voltage across the voltage source and I is the current that flows through the voltage source)