diff -r 810413b745ff -r 9966f44ecab4 mercurial/debugcommands.py
--- a/mercurial/debugcommands.py	Fri Apr 06 10:46:24 2018 -0700
+++ b/mercurial/debugcommands.py	Fri Apr 06 11:26:50 2018 -0700
@@ -2547,7 +2547,7 @@
 @command('debugwhyunstable', [], _('REV'))
 def debugwhyunstable(ui, repo, rev):
     """explain instabilities of a changeset"""
-    for entry in obsutil.whyunstable(repo, repo[rev]):
+    for entry in obsutil.whyunstable(repo, scmutil.revsingle(repo, rev)):
         dnodes = ''
         if entry.get('divergentnodes'):
             dnodes = ' '.join('%s (%s)' % (ctx.hex(), ctx.phasestr())