diff --git a/lib/flutter_user_agent.dart b/lib/flutter_user_agent.dart
index 11312ef..da4bf18 100644
--- a/lib/flutter_user_agent.dart
+++ b/lib/flutter_user_agent.dart
@@ -5,7 +5,7 @@ import 'package:flutter/services.dart';
 class FlutterUserAgent {
   static const MethodChannel _channel = MethodChannel('flutter_user_agent');
 
-  static Map<String, dynamic> _properties;
+  static Map<String, dynamic>? _properties;
 
   /// Initialize the module.
   ///
@@ -27,28 +27,28 @@ class FlutterUserAgent {
   }
 
   /// Returns the device's user agent.
-  static String get userAgent {
-    return _properties['userAgent'];
+  static String? get userAgent {
+    return _properties?['userAgent'];
   }
 
   /// Returns the device's webview user agent.
-  static String get webViewUserAgent {
-    return _properties['webViewUserAgent'];
+  static String? get webViewUserAgent {
+    return _properties?['webViewUserAgent'];
   }
 
   /// Fetch a [property] that can be used to build your own user agent string.
   static dynamic getProperty(String property) {
-    return _properties[property];
+    return _properties?[property];
   }
 
   /// Fetch a [property] asynchronously that can be used to build your own user agent string.
   static dynamic getPropertyAsync(String property) async {
     await init();
-    return _properties[property];
+    return _properties?[property];
   }
 
   /// Return a map of properties that can be used to generate the user agent string.
-  static Map<String, dynamic> get properties {
+  static Map<String, dynamic>? get properties {
     return _properties;
   }
 }
diff --git a/pubspec.yaml b/pubspec.yaml
index 4617469..9f085d2 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -6,7 +6,7 @@ version: 1.2.2
 homepage: https://github.com/j0j00/flutter_user_agent
 
 environment:
-  sdk: ">=2.0.0-dev.68.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter: