diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -137,10 +137,11 @@ class Server(NetNode):
 		for (i,k) in enumerate(indices):
 			if self._lastIndex+1!=k:
 				self._dataFile.seek(k*self.BLOCK_SIZE)
-			self._dataFile.write(binData[i*self.BLOCK_SIZE:(i+1)*self.BLOCK_SIZE])
+			block=binData[i*self.BLOCK_SIZE:(i+1)*self.BLOCK_SIZE]
+			self._dataFile.write(block)
 			self._lastIndex=k
 			if self._treeFile:
-				self._newLeaves[k+self._tree.leafStart]=hashBlock(binData)
+				self._newLeaves[k+self._tree.leafStart]=hashBlock(block)
 
 		return ({"command": "ack", "index": indices},)