Binding of Isaac: Rebirth Wiki

Undefined is an unlockable activated item.




  • The I AM ERROR I AM ERROR room in Cathedral Cathedral and Sheol Sheol will have a beam of light/trap door leading to the next floor, even if Isaac doesn't have The Polaroid The Polaroid or The Negative The Negative.
  • Removed in Afterbirth Chest Chest and Dark Room Dark Room are reset on exiting the I AM ERROR room, which can be done repeatedly as long as the player can gather enough charges to successfully warp to it. This can provide a ridiculous number of items from the golden chests in Chest Chest.
    • This makes it possible to kill ??? ??? or The Lamb The Lamb multiple times. If Isaac has an item that resurrects him as a different character, multiple achievements and items can be unlocked in a single run.
    • Added in Afterbirth † The Void The Void is also reset and can be cleared repeatedly.
  • Added in Repentance As no Secret Rooms or Super Secret Rooms spawn on any floor during the Ascent, this item will only warp Isaac to either the Treasure Room or the I AM ERROR room. This can be a reliable method of gaining many items on the way up to Home Home, while also allowing the player to skip potentially difficult encounters.

In-game Footage[]


  • The item is a reference to a bug in the original The Binding of Isaac, where exhausting an Item Pool (or rarely simply by chance) would cause an item to spawn whose name and description was listed as "Undefined", and whose sprite would rapidly cycle through the sprites of every other item in the game. Undefined by itself creates no effect, but if it was ever picked up, should the game attempt to spawn an item from an exhausted item pool, the game instead crashed, as it could no longer spawn Undefined. From Rebirth onwards, an exhausted item pool will instead spawn Breakfast Breakfast, even if the item would normally never spawn from that item pool and/or Breakfast had already been obtained in the run.
  • In programming "Undefined" refers to an unknown value.
  • Given the Glitch theme of the item and the ability to access Chapters that have not yet been unlocked using I AM ERROR I AM ERROR, along with the ability to acquire mass upgrades in The Chest and The Dark Room, it is possible that it was intentionally made into a powerful sequence-breaking item.
  • The sprite for Undefined appears to be, fittingly, a glitched and spliced version of other item sprites, with the bottom right part noticeably being the The Compass The Compass, the bottom left being the Blood Bag Blood Bag, and the top half being Contract From Below Contract From Below. This may also be a reference to Undefined's sprite in the original The Binding of Isaac.



PC TRP0 FE16 (first treasure room, adjacent to spawn)

PC 9YDH V6SL (first treasure room, adjacent to spawn)

PC F189 W42E (first treasure room, adjacent to spawn. Teleports to I AM ERROR room upon first use in hard mode.)

PC 0JGY 6ZWX (first treasure room near spawn)

PC W0WZ QCRJ (first treasure room, adjacent to spawn)

PC 6AGC GCCH (first treasure room, adjacent to spawn)

PS4 SJKJ XZB2 (first treasure room, adjacent to spawn)

Vita 4CG9 42ED (first treasure room, adjacent to spawn)

3DS YAD2 KJCM (first treasure room, adjacent to spawn)

3DS 8LQF 7MT0 (first floor treasure room)