diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
index d47a61d170..2bd6f36d90 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.js
@@ -632,7 +632,7 @@
     _createThreadElement(thread) {
       const threadEl = document.createElement('gr-comment-thread');
       threadEl.className = 'comment-thread';
-      threadEl.slot = `${thread.commentSide}-${thread.lineNum}`;
+      threadEl.setAttribute('slot', `${thread.commentSide}-${thread.lineNum}`);
       threadEl.comments = thread.comments;
       threadEl.commentSide = thread.commentSide;
       threadEl.isOnParent = !!thread.isOnParent;
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index d7e193c750..d2f9214ceb 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -775,7 +775,7 @@
           // that is okay because the first matching slot is used and the rest
           // are ignored.
           const slot = document.createElement('slot');
-          slot.name = threadEl.slot;
+          slot.name = threadEl.getAttribute('slot');
           Polymer.dom(threadGroupEl).appendChild(Gerrit.slotToContent(slot));
         }
       });