diff --git a/lib/mop.dart b/lib/mop.dart
index eb42e08..c2cb6b0 100644
--- a/lib/mop.dart
+++ b/lib/mop.dart
@@ -919,6 +919,8 @@ class Mop {
 
   final Map<String, ExtensionApiHandler> _extensionApis = {};
 
+  final Map<String, SyncExtensionApiHandler> _syncExtensionApis = {};
+
   Map<String, ExtensionApiHandler> _webExtensionApis = {};
 
   factory Mop() {