diff --git a/hgext/fastannotate/protocol.py b/hgext/fastannotate/protocol.py
--- a/hgext/fastannotate/protocol.py
+++ b/hgext/fastannotate/protocol.py
@@ -76,7 +76,7 @@
                     content = f.read()
                 vfsbaselen = len(repo.vfs.base + '/')
                 relpath = p[vfsbaselen:]
-                result += '%s\0%s\0%s' % (relpath, len(content), content)
+                result += '%s\0%d\0%s' % (relpath, len(content), content)
     return result
 
 def _registerwireprotocommand():