diff --git a/mercurial/bookmarks.py b/mercurial/bookmarks.py
--- a/mercurial/bookmarks.py
+++ b/mercurial/bookmarks.py
@@ -28,6 +28,9 @@ def read(repo):
         for line in repo.opener('bookmarks'):
             line = line.strip()
             if ' ' not in line:
+                if line:
+                    msg = _('malformed line in .hg/bookmarks: %r\n')
+                    repo.ui.warn( msg % line)
                 continue
             sha, refspec = line.strip().split(' ', 1)
             refspec = encoding.tolocal(refspec)
diff --git a/tests/test-bookmarks.t b/tests/test-bookmarks.t
--- a/tests/test-bookmarks.t
+++ b/tests/test-bookmarks.t
@@ -350,3 +350,11 @@ test wrongly formated bookmark
      Y                         2:db815d6d32e6
    * Z                         3:125c9a1d6df6
      x  y                      2:db815d6d32e6
+  $ echo "Ican'thasformatedlines" >> .hg/bookmarks
+  $ hg bookmarks
+  malformed line in .hg/bookmarks: "Ican'thasformatedlines"
+     X2                        1:925d80f479bb
+     Y                         2:db815d6d32e6
+   * Z                         3:125c9a1d6df6
+     x  y                      2:db815d6d32e6
+