diff --git a/mclevel.py b/mclevel.py
index 7e2fc4f..24689e2 100644
--- a/mclevel.py
+++ b/mclevel.py
@@ -459,6 +459,8 @@ class MCLevel(object):
         return self.Blocks[x:x+w,z:z+l,y:y+h]
     
     def fillBlocks(self, box, blockType, blockData = 0, blocksToReplace = None):
+        if box == None: box = self.getWorldBounds()
+        
         info("Filling blocks in {0} with {1}, data={2} replacing{3}".format(box, blockType, blockData, blocksToReplace) )
         slices = map(slice, box.origin, box.maximum)