Binding of Isaac: Rebirth Wiki

Undefined is an unlockable activated item.

Effects[ | ]

Strategies[ | ]

  • The I AM ERRORI AM ERROR room in CathedralCathedral and SheolSheol will have a beam of light/trap door leading to the next floor, even if Isaac doesn't have Collectible The Polaroid iconThe Polaroid or Collectible The Negative iconThe Negative.
  • (in Rebirth) ChestChest and Dark RoomDark 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 ChestChest.
    • This makes it possible to kill Boss ??? ingame??? or Boss The Lamb ingameThe 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.
    • (in Afterbirth † and Repentance)The VoidThe Void is also reset and can be cleared repeatedly.
  • (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 HomeHome, while also allowing the player to skip potentially difficult encounters.
  • (in Repentance) Has no effect when used in HomeHome.
  • Using Undefined on (except in Rebirth)?????? always sends the player to the I AM ERROR room. The Error room will always contain a trap door, not a lightbeam, so it will always send the player to Sheol.

Synergies[ | ]

Interactions[ | ]

In-game Footage[ | ]

Trivia[ | ]

  • 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 iconBreakfast, 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 ERRORI 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 iconThe Compass, the bottom left being the Collectible Blood Bag iconBlood Bag, and the top half being Collectible Contract From Below iconContract From Below. This may also be a reference to Undefined's sprite in the original The Binding of Isaac.

Gallery[ | ]