--- a/net/minecraft/server/TileEntityPiston.java
+++ b/net/minecraft/server/TileEntityPiston.java
@@ -132,6 +132,7 @@
     }
 
     public void c() {
+        if (this.world == null) return; // CraftBukkit
         this.j = this.i;
         if (this.j >= 1.0F) {
             this.j();