--- a/net/minecraft/network/syncher/DataWatcher.java
+++ b/net/minecraft/network/syncher/DataWatcher.java
@@ -142,6 +142,13 @@
 
     }
 
+    // CraftBukkit start - add method from above
+    public <T> void markDirty(DataWatcherObject<T> datawatcherobject) {
+        this.getItem(datawatcherobject).setDirty(true);
+        this.isDirty = true;
+    }
+    // CraftBukkit end
+
     public boolean isDirty() {
         return this.isDirty;
     }
@@ -274,7 +281,7 @@
         if (!Objects.equals(datawatcher_item1.accessor.getSerializer(), datawatcher_item.accessor.getSerializer())) {
             throw new IllegalStateException(String.format(Locale.ROOT, "Invalid entity data item type for field %d on entity %s: old=%s(%s), new=%s(%s)", datawatcher_item.accessor.getId(), this.entity, datawatcher_item.value, datawatcher_item.value.getClass(), datawatcher_item1.value, datawatcher_item1.value.getClass()));
         } else {
-            datawatcher_item.setValue(datawatcher_item1.getValue());
+            datawatcher_item.setValue((T) datawatcher_item1.getValue()); // CraftBukkit - decompile error
         }
     }