Undefined is an unlockable activated item.



  • The I AM ERROR room in Stage Cathedral icon.png Cathedral and Stage Sheol icon.png Sheol will have a beam of light/trap door leading to the next floor, even if Isaac doesn't have Collectible The Polaroid icon.png The Polaroid or Collectible The Negative icon.png The Negative.
  • Dlc na indicator.png Stage Chest icon.png Chest and Stage Dark Room icon.png 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 Stage Chest icon.png Chest.
    • This makes it possible to kill ??? or 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.
    • Dlc a† indicator.png Stage The Void icon.png The Void is also reset and can be cleared repeatedly.
  • Dlc r indicator.png 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 Stage Home icon.png 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 Collectible Breakfast icon.png 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, 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 Collectible The Compass icon.png The Compass, the bottom left being the Collectible Blood Bag icon.png Blood Bag, and the top half being Collectible Contract From Below icon.png 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)

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