diff --git a/test/async_cache_test.dart b/test/async_cache_test.dart
index 92e34bf..5474a37 100644
--- a/test/async_cache_test.dart
+++ b/test/async_cache_test.dart
@@ -22,7 +22,7 @@
 
   test('should not fetch via callback when a cache exists', () async {
     await cache.fetch(() async => 'Expensive');
-    expect(await cache.fetch(expectAsync0(() {}, count: 0)), 'Expensive');
+    expect(await cache.fetch(expectAsync0(() => null, count: 0)), 'Expensive');
   });
 
   test('should not fetch via callback when a future is in-flight', () async {
@@ -31,7 +31,7 @@
 
     var completer = Completer<String>();
     expect(cache.fetch(() => completer.future), completion('Expensive'));
-    expect(cache.fetch(expectAsync0(() {}, count: 0)), completion('Expensive'));
+    expect(cache.fetch(expectAsync0(() => null, count: 0)), completion('Expensive'));
     completer.complete('Expensive');
   });
 
@@ -78,7 +78,7 @@
       yield '2';
       yield '3';
     }).toList();
-    expect(await cache.fetchStream(expectAsync0(() {}, count: 0)).toList(),
+    expect(await cache.fetchStream(expectAsync0(() => null, count: 0)).toList(),
         ['1', '2', '3']);
   });
 
diff --git a/test/cancelable_operation_test.dart b/test/cancelable_operation_test.dart
index a4c9308..a47e952 100644
--- a/test/cancelable_operation_test.dart
+++ b/test/cancelable_operation_test.dart
@@ -251,9 +251,9 @@
 
     setUp(() {
       // Initialize all functions to ones that expect to not be called.
-      onValue = expectAsync1((_) {}, count: 0, id: "onValue");
-      onError = expectAsync2((e, s) {}, count: 0, id: "onError");
-      onCancel = expectAsync0(() {}, count: 0, id: "onCancel");
+      onValue = expectAsync1((_) => null, count: 0, id: "onValue");
+      onError = expectAsync2((e, s) => null, count: 0, id: "onError");
+      onCancel = expectAsync0(() => null, count: 0, id: "onCancel");
       propagateCancel = false;
     });
 
diff --git a/test/stream_queue_test.dart b/test/stream_queue_test.dart
index aaf1227..b2a8625 100644
--- a/test/stream_queue_test.dart
+++ b/test/stream_queue_test.dart
@@ -254,6 +254,7 @@
             expect(value, expectedValue);
             expect(index, sequenceIndex);
             index++;
+            return null;
           };
       await Future.wait([
         skip1.then(sequence(0, 0)),
diff --git a/test/subscription_transformer_test.dart b/test/subscription_transformer_test.dart
index 2a709a4..64f45ec 100644
--- a/test/subscription_transformer_test.dart
+++ b/test/subscription_transformer_test.dart
@@ -90,6 +90,7 @@
           subscriptionTransformer(handleCancel: expectAsync1((inner) {
         callbackInvoked = true;
         inner.cancel();
+        return null;
       }))).listen(expectAsync1((_) {}, count: 0));
 
       await flushMicrotasks();
@@ -251,7 +252,7 @@
     setUp(() {
       var controller = StreamController();
       subscription = controller.stream
-          .transform(subscriptionTransformer(handleCancel: (_) {}))
+          .transform(subscriptionTransformer(handleCancel: (_) => null))
           .listen(expectAsync1((_) {}, count: 0),
               onError: expectAsync2((_, __) {}, count: 0),
               onDone: expectAsync0(() {}, count: 0));