diff --git a/lib/cmdlib.py b/lib/cmdlib.py
index fbe494c5e5f9b8ace917be152f7595d6324d4ed1..a47c9ee0fa2eea86ffc645ccf7b7eff7b38f9291 100644
--- a/lib/cmdlib.py
+++ b/lib/cmdlib.py
@@ -6364,7 +6364,7 @@ class TLReplaceDisks(Tasklet):
     """
     self.instance = instance = self.cfg.GetInstanceInfo(self.instance_name)
     assert instance is not None, \
-      "Cannot retrieve locked instance %s" % instance_name
+      "Cannot retrieve locked instance %s" % self.instance_name
 
     if instance.disk_template != constants.DT_DRBD8:
       raise errors.OpPrereqError("Can only run replace disks for DRBD8-based"