diff --git a/lib/media/source_buffer_manager.js b/lib/media/source_buffer_manager.js
index 72d8b21db6..f91666d8b8 100644
--- a/lib/media/source_buffer_manager.js
+++ b/lib/media/source_buffer_manager.js
@@ -647,7 +647,11 @@ shaka.media.SourceBufferManager.prototype.onSourceBufferUpdateEnd_ =
   shaka.log.v1(this.logPrefix_(), 'onSourceBufferUpdateEnd_');
 
   shaka.asserts.assert(!this.sourceBuffer_.updating);
-  shaka.asserts.assert(this.operationPromise_);
+  if (!this.operationPromise_) {
+    // Spurious event, which seems to happen on IE11 with rapid seeking.
+    // Ignore.
+    return;
+  }
 
   this.operationPromise_.resolve();
   this.operationPromise_ = null;