diff --git a/lib/VirtSubproc.py b/lib/VirtSubproc.py
index e674080..4eb9079 100644
--- a/lib/VirtSubproc.py
+++ b/lib/VirtSubproc.py
@@ -470,7 +470,7 @@ def copyupdown_internal(wh, sd, upp):
bomb("%s paths must be nonempty" % wh)
dirsp = sd[0][-1] == '/'
if dirsp != (sd[1][-1] == '/'):
- bomb("% paths must agree about directoryness"
+ bomb("%s paths must agree about directoryness"
" (presence or absence of trailing /)" % wh)
# if we have a shared directory, we just need to copy it from/to there; in