diff --git a/src/static/js/Changeset.js b/src/static/js/Changeset.js
index 85ee35700..426835915 100644
--- a/src/static/js/Changeset.js
+++ b/src/static/js/Changeset.js
@@ -488,9 +488,11 @@ exports.opAssembler = () => {
    * @param {Op} op - Operation to add. Ownership remains with the caller.
    */
   const append = (op) => {
-    if (op.attribs != null) serialized += op.attribs;
+    if (!op.opcode) throw new TypeError('null op');
+    if (typeof op.attribs !== 'string') throw new TypeError('attribs must be a string');
+    serialized += op.attribs;
     if (op.lines) serialized += `|${exports.numToString(op.lines)}`;
-    if (op.opcode != null) serialized += op.opcode;
+    serialized += op.opcode;
     serialized += exports.numToString(op.chars);
   };
 
diff --git a/src/static/js/contentcollector.js b/src/static/js/contentcollector.js
index 3d2bd9aa8..54c628804 100644
--- a/src/static/js/contentcollector.js
+++ b/src/static/js/contentcollector.js
@@ -92,7 +92,7 @@ const makeContentCollector = (collectStyles, abrowser, apool, className2Author)
         attribsBuilder = Changeset.smartOpAssembler();
       },
       textOfLine: (i) => textArray[i],
-      appendText: (txt, attrString) => {
+      appendText: (txt, attrString = '') => {
         textArray[textArray.length - 1] += txt;
         op.attribs = attrString;
         op.chars = txt.length;