Can exhausted units be chosen to defend against attackers?

RRG, p. 26:

"2) Declare defender. The attacking player chooses an enemy unit at the planet, and declares this attack is against it. This declares the chosen unit as the 'defender.'"

Since there is nothing in there limiting the choice to ready units, any enemy unit at the planet - ready or exhausted - may be chosen as a defender.

Thank you!