diff --git a/lib/ports.dart b/lib/ports.dart
index ec3c55e..3fcdcb3 100644
--- a/lib/ports.dart
+++ b/lib/ports.dart
@@ -267,7 +267,7 @@
 /// The first value sent to [port] is used to complete the [result].
 /// All following values sent to `port` are ignored.
 class SingleResponseChannel<R> {
-  Zone _zone;
+  final Zone _zone;
   final RawReceivePort _receivePort;
   final Completer<R> _completer;
   final Function _callback;
diff --git a/lib/registry.dart b/lib/registry.dart
index 5b681dd..9fbfbc2 100644
--- a/lib/registry.dart
+++ b/lib/registry.dart
@@ -101,7 +101,7 @@
   static final Expando _caches = Expando();
 
   /// Port for sending command to the central registry manager.
-  SendPort _commandPort;
+  final SendPort _commandPort;
 
   /// Create a registry linked to a [RegistryManager] through [commandPort].
   ///
@@ -329,8 +329,8 @@
 /// The central repository used by distributed [Registry] instances.
 class RegistryManager {
   final Duration _timeout;
+  final RawReceivePort _commandPort;
   int _nextId = 0;
-  RawReceivePort _commandPort;
 
   /// Maps id to entry. Each entry contains the id, the element, its tags,
   /// and a capability required to remove it again.