diff --git a/lib/src/voip_content.dart b/lib/src/voip_content.dart
index 6f34969b..1ac24ebf 100644
--- a/lib/src/voip_content.dart
+++ b/lib/src/voip_content.dart
@@ -1,8 +1,25 @@
-// @dart=2.9
+/*
+ *   Famedly Matrix SDK
+ *   Copyright (C) 2021 Famedly GmbH
+ *
+ *   This program is free software: you can redistribute it and/or modify
+ *   it under the terms of the GNU Affero General Public License as
+ *   published by the Free Software Foundation, either version 3 of the
+ *   License, or (at your option) any later version.
+ *
+ *   This program is distributed in the hope that it will be useful,
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ *   GNU Affero General Public License for more details.
+ *
+ *   You should have received a copy of the GNU Affero General Public License
+ *   along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
 class CallReplacesTarget {
-  String id;
-  String display_name;
-  String avatar_url;
+  String? id;
+  String? display_name;
+  String? avatar_url;
 
   CallReplacesTarget({this.id, this.display_name, this.avatar_url});
   factory CallReplacesTarget.fromJson(Map<String, dynamic> json) =>
@@ -22,11 +39,11 @@ class CallReplacesTarget {
 /// MSC2747: VoIP call transfers
 /// https://github.com/matrix-org/matrix-doc/pull/2747
 class CallReplaces {
-  String replacement_id;
-  CallReplacesTarget target_user;
-  String create_call;
-  String await_call;
-  String target_room;
+  String? replacement_id;
+  CallReplacesTarget? target_user;
+  String? create_call;
+  String? await_call;
+  String? target_room;
 
   CallReplaces({
     this.replacement_id,
@@ -36,16 +53,16 @@ class CallReplaces {
     this.target_room,
   });
   factory CallReplaces.fromJson(Map<String, dynamic> json) => CallReplaces(
-        replacement_id: json['replacement_id'].toString(),
-        create_call: json['create_call'].toString(),
-        await_call: json['await_call'].toString(),
-        target_room: json['target_room'].toString(),
+        replacement_id: json['replacement_id']?.toString(),
+        create_call: json['create_call']?.toString(),
+        await_call: json['await_call']?.toString(),
+        target_room: json['target_room']?.toString(),
         target_user: CallReplacesTarget.fromJson(json['target_user']),
       );
 
   Map<String, dynamic> toJson() => {
         if (replacement_id != null) 'replacement_id': replacement_id,
-        if (target_user != null) 'target_user': target_user.toJson(),
+        if (target_user != null) 'target_user': target_user!.toJson(),
         if (create_call != null) 'create_call': create_call,
         if (await_call != null) 'await_call': await_call,
         if (target_room != null) 'target_room': target_room,
@@ -60,11 +77,11 @@ const String sdpStreamMetadataKey = 'org.matrix.msc3077.sdp_stream_metadata';
 class CallCapabilities {
   bool transferee;
   bool dtmf;
-  CallCapabilities({this.transferee, this.dtmf});
+  CallCapabilities({this.transferee = false, this.dtmf = false});
   factory CallCapabilities.fromJson(Map<String, dynamic> json) =>
       CallCapabilities(
-        dtmf: json['m.call.dtmf'] as bool ?? false,
-        transferee: json['m.call.transferee'] as bool ?? false,
+        dtmf: json['m.call.dtmf'] as bool? ?? false,
+        transferee: json['m.call.transferee'] as bool? ?? false,
       );
   Map<String, dynamic> toJson() => {
         if (transferee != null) 'm.call.transferee': transferee,
@@ -88,11 +105,14 @@ class SDPStreamPurpose {
   bool audio_muted;
   bool video_muted;
 
-  SDPStreamPurpose({this.purpose, this.audio_muted, this.video_muted});
+  SDPStreamPurpose(
+      {required this.purpose,
+      this.audio_muted = false,
+      this.video_muted = false});
   factory SDPStreamPurpose.fromJson(Map<String, dynamic> json) =>
       SDPStreamPurpose(
-        audio_muted: json['audio_muted'] as bool ?? false,
-        video_muted: json['video_muted'] as bool ?? false,
+        audio_muted: json['audio_muted'] as bool? ?? false,
+        video_muted: json['video_muted'] as bool? ?? false,
         purpose: json['purpose'] as String,
       );
 
@@ -122,15 +142,15 @@ class SDPStreamMetadata {
 /// MSC3086: Asserted identity on VoIP calls
 /// https://github.com/matrix-org/matrix-doc/pull/3086
 class AssertedIdentity {
-  String id;
-  String displayName;
-  String avatarUrl;
+  String? id;
+  String? displayName;
+  String? avatarUrl;
   AssertedIdentity({this.id, this.displayName, this.avatarUrl});
   factory AssertedIdentity.fromJson(Map<String, dynamic> json) =>
       AssertedIdentity(
-        displayName: json['display_name'] as String,
-        id: json['id'] as String,
-        avatarUrl: json['avatar_url'] as String,
+        displayName: json['display_name'] as String?,
+        id: json['id'] as String?,
+        avatarUrl: json['avatar_url'] as String?,
       );
   Map<String, dynamic> toJson() => {
         if (displayName != null) 'display_name': displayName,