diff --git a/mercurial/phases.py b/mercurial/phases.py
--- a/mercurial/phases.py
+++ b/mercurial/phases.py
@@ -148,8 +148,9 @@ def filterunknown(repo, phaseroots=None)
         missing = [node for node in nodes if node not in nodemap]
         if missing:
             for mnode in missing:
-                msg = 'Removing unknown node %(n)s from %(p)i-phase boundary'
-                repo.ui.debug(msg, {'n': short(mnode), 'p': phase})
+                repo.ui.debug(
+                    'removing unknown node %s from %i-phase boundary\n'
+                    % (short(mnode), phase))
             nodes.symmetric_difference_update(missing)
             repo._dirtyphases = True