It seems this isn’t a rule that’s been uploaded yet.
From my French textbook:
Lorsqu’un adjectif est employé adverbialement, il reste invariable. (Pour savoir si un adjectif est employé ainsi, on peut le remplacer par un adverbe en -ment, comme gentiment, doucement, malheureusement…).
When an adjective is used as an adverb / adverbially, it remains invariable. To know if an adjective is used this way, we can replace it by an adverb ending in -ment (ly), like gently, lightly, unfortunately, etc.
Les adjectifs qui peuvent être employés adverbialement sont : (the adjectives that can be used as an adverb / adverbially):
bas, bon, cher, claire, court, creux, doux, droit, dru, dur, faux, ferme, fin, fort, franc, gras, gros, haut, juste, lourd, mauvais, net, profond, sec, vrai.