diff --git a/lib/src/widgets/controller.dart b/lib/src/widgets/controller.dart
index b3173ed2..15104add 100644
--- a/lib/src/widgets/controller.dart
+++ b/lib/src/widgets/controller.dart
@@ -85,7 +85,7 @@ class QuillController extends ChangeNotifier {
   }
 
   void _handleHistoryChange(int? len) {
-    if (len != 0) {
+    if (len! != 0) {
       // if (this.selection.extentOffset >= document.length) {
       // // cursor exceeds the length of document, position it in the end
       // updateSelection(