diff --git a/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/levels/PrisonBossLevel.java b/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/levels/PrisonBossLevel.java
index b5c72876e..98a5a0c69 100644
--- a/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/levels/PrisonBossLevel.java
+++ b/core/src/main/java/com/shatteredpixel/shatteredpixeldungeon/levels/PrisonBossLevel.java
@@ -649,7 +649,7 @@ public class PrisonBossLevel extends Level {
 		Painter.fill(this, tenguCell, 1, Terrain.EMPTY);
 		buildFlagMaps();
 
-		for (CustomTilemap vis : customTiles){
+		for (CustomTilemap vis : customTiles.toArray(new CustomTilemap[0])){
 			if (vis instanceof FadingTraps){
 				((FadingTraps) vis).remove();
 			}