diff --git a/mercurial/commands.py b/mercurial/commands.py
--- a/mercurial/commands.py
+++ b/mercurial/commands.py
@@ -7077,7 +7077,8 @@ def tag(ui, repo, name1, *names, **opts)
                 raise error.Abort(_('uncommitted merge'))
             bheads = repo.branchheads()
             if not opts.get('force') and bheads and p1 not in bheads:
-                raise error.Abort(_('not at a branch head (use -f to force)'))
+                raise error.Abort(_('working directory is not at a branch head '
+                                    '(use -f to force)'))
         r = scmutil.revsingle(repo, rev_).node()
 
         if not message:
diff --git a/tests/test-tag.t b/tests/test-tag.t
--- a/tests/test-tag.t
+++ b/tests/test-tag.t
@@ -109,7 +109,7 @@ tagging on a non-head revision
   0 files updated, 0 files merged, 1 files removed, 0 files unresolved
   $ hg tag -l localblah
   $ hg tag "foobar"
-  abort: not at a branch head (use -f to force)
+  abort: working directory is not at a branch head (use -f to force)
   [255]
   $ hg tag -f "foobar"
   $ cat .hgtags
@@ -339,7 +339,7 @@ tagging on null rev
   $ hg up null
   0 files updated, 0 files merged, 2 files removed, 0 files unresolved
   $ hg tag nullrev
-  abort: not at a branch head (use -f to force)
+  abort: working directory is not at a branch head (use -f to force)
   [255]
 
   $ hg init empty