diff --git a/borg/archive.py b/borg/archive.py
index 0ac565c4a..31e3c0571 100644
--- a/borg/archive.py
+++ b/borg/archive.py
@@ -95,7 +95,7 @@ def flush(self, flush=False):
         for chunk in chunks[:end]:
             self.chunks.append(self.write_chunk(chunk))
         if end == -1:
-            self.buffer.write(chunks[-1])
+            self.buffer.write(chunks[-1].data)
 
     def is_full(self):
         return self.buffer.tell() > self.BUFFER_SIZE