diff --git a/lib/utils/flattened-nodes-observer.js b/lib/utils/flattened-nodes-observer.js
index 87802d5335..dbbd05d40f 100644
--- a/lib/utils/flattened-nodes-observer.js
+++ b/lib/utils/flattened-nodes-observer.js
@@ -252,7 +252,8 @@ export class FlattenedNodesObserver {
       addedNodes: [],
       removedNodes: []
     };
-    let newNodes = this.constructor.getFlattenedNodes(this._target);
+    let newNodes = this.constructor.getFlattenedNodes(
+        /** @type {!HTMLElement} */ (this._target));
     let splices = calculateSplices(newNodes,
       this._effectiveNodes);
     // process removals