diff --git a/nw/core/index.py b/nw/core/index.py
index 2299ab0d..73d469c4 100644
--- a/nw/core/index.py
+++ b/nw/core/index.py
@@ -714,7 +714,8 @@ class NWIndex():
         for refTitle in self._refIndex[tHandle]:
             for aTag in self._refIndex[tHandle][refTitle].get("tags", []):
                 if len(aTag) == 3 and (sTitle is None or sTitle == refTitle):
-                    theRefs[aTag[1]].append(aTag[2])
+                    if aTag[1] in theRefs: # Future-compatible. Check can be removed in 1.2.
+                        theRefs[aTag[1]].append(aTag[2])
 
         return theRefs