--- ../src-base/minecraft/net/minecraft/tileentity/TileEntityPiston.java
+++ ../src-work/minecraft/net/minecraft/tileentity/TileEntityPiston.java
@@ -140,6 +140,11 @@
 
     public void updateEntity()
     {
+        if (this.worldObj == null)
+        {
+            return;    // CraftBukkit
+        }
+
         this.lastProgress = this.progress;
 
         if (this.lastProgress >= 1.0F)
