Looks like I found a bug.

[Euphytose]

The orb of light needs 40 mana to be casted, but even if you have less than that, it will work.

It won't use any mana, so even if you have 0 mana max it works, and the casting sound won't play.

Nevermind...

When summoned, the Orb of Light will follow the player around and project light in a manner similar to the fairy bell. However, the Orb of Light has low acceleration and a slow top speed, so it often falls behind, especially when the player falls a long distance or otherwise travels at a fast speed (such as when using the Hermes/Spectre Boots. If it falls too far behind it will warp to the player's location. The Orb of Light costs 40 mana to summon and will follow the character for exactly five minutes, then disappear. If your maximum mana is below 40 however, it will be free to cast, yet is not usable by hotkey.