When I add an Armature with a single Bone to the scene, like so:

and then an Empty object, as follows:

and then I add a Copy Rotation bone-constraint, where I set the Empty object as the target:

the constraint will rotate my bone on the x axis by 90 degree:

Although neither the Armature, nor the Bone and not even the Empty has any rotations whatsoever..

My question is: why is this happening and if this is the expected behaviour, then how can I prevent it from happening?