diff --git a/src/static/js/Changeset.js b/src/static/js/Changeset.js
index f9270385..5841dadd 100644
--- a/src/static/js/Changeset.js
+++ b/src/static/js/Changeset.js
@@ -1504,6 +1504,7 @@ exports.moveOpsToNewPool = function (cs, oldPool, newPool) {
   return upToDollar.replace(/\*([0-9a-z]+)/g, function (_, a) {
     var oldNum = exports.parseNum(a);
     var pair = oldPool.getAttrib(oldNum);
+    if(!pair) exports.error('Can\'t copy unknown attrib (reference attrib string to non-existant pool entry). Inconsistent attrib state!');
     var newNum = newPool.putAttrib(pair);
     return '*' + exports.numToString(newNum);
   }) + fromDollar;