diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 9912e638222..1ca0956cd6e 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -16966,8 +16966,8 @@ class Geolocation extends JavaScriptObject {
         new StreamController<Geoposition>(
             sync: true,
             onCancel: () {
-              assert(watchId != null);
-              _clearWatch(watchId!);
+              final id = watchId;
+              if (id != null) _clearWatch(id);
             });
     controller.onListen = () {
       assert(watchId == null);
diff --git a/tools/dom/templates/html/impl/impl_Geolocation.darttemplate b/tools/dom/templates/html/impl/impl_Geolocation.darttemplate
index db6fbbf5c4b..bee8d856487 100644
--- a/tools/dom/templates/html/impl/impl_Geolocation.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Geolocation.darttemplate
@@ -52,8 +52,8 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
     StreamController<Geoposition> controller =
       new StreamController<Geoposition>(sync: true,
         onCancel: () {
-          assert(watchId != null);
-          _clearWatch(watchId$NULLASSERT);
+          final id = watchId;
+          if (id != null) _clearWatch(id);
         }
       );
     controller.onListen = () {