Each Axis is represented with a different enumerator, with a unique number (https://github.com/SFML/SFML/blob/master/include/SFML/Window/Joystick.hpp#L60-L70):
enum Axis
{
X, ///< The X axis
Y, ///< The Y axis
Z, ///< The Z axis
R, ///< The R axis
U, ///< The U axis
V, ///< The V axis
PovX, ///< The X axis of the point-of-view hat
PovY ///< The Y axis of the point-of-view hat
};
Joystick buttons on the other hand are integers, indexed starting from 0. You can easily solve your problem by not mixing Axis and Button -- this is the reason why different types are used to represent both in SFML.
Of course, if you need to have a single address space to represent both axis and button (e.g. for serialization), it should be an easy exercise to achieve that in client code :)
By the way, in case you use Thor, I'm providing functions to serialize joystick axes from/to strings. This would allow you to display this in the game's UI. The documentation can be found here (http://www.bromeon.ch/libraries/thor/documentation/v2.1/_input_names_8hpp.html).