Index: sphinx/directives.py
===================================================================
--- sphinx/directives.py	(revision 62221)
+++ sphinx/directives.py	(working copy)
@@ -646,7 +646,7 @@
         docname = posixpath.normpath(posixpath.join(dirname, docname))
         if docname not in env.found_docs:
             ret.append(state.document.reporter.warning(
-                'toctree references unknown document %s' % docname, line=lineno))
+                "toctree references unknown document '%s'" % docname, line=lineno))
         else:
             includefiles.append(docname)
     subnode['includefiles'] = includefiles