diff --git a/sphinx/environment.py b/sphinx/environment.py
index d3e3196ba..d22cf5e49 100644
--- a/sphinx/environment.py
+++ b/sphinx/environment.py
@@ -1235,7 +1235,8 @@ class BuildEnvironment:
                     # keywords are referenced by named labels
                     docname, labelid, _ = self.labels.get(target, ('','',''))
                     if not docname:
-                        #self.warn(node['refdoc'], 'unknown keyword: %s' % target)
+                        #self.warn(node['refdoc'],
+                        #          'unknown keyword: %s' % target)
                         newnode = contnode
                     else:
                         newnode = nodes.reference('', '')