diff --git a/src/core/Akka.Streams.Tests/Dsl/FlowInterleaveSpec.cs b/src/core/Akka.Streams.Tests/Dsl/FlowInterleaveSpec.cs
index c692182603e..3bd31bf16cc 100644
--- a/src/core/Akka.Streams.Tests/Dsl/FlowInterleaveSpec.cs
+++ b/src/core/Akka.Streams.Tests/Dsl/FlowInterleaveSpec.cs
@@ -9,6 +9,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
+using System.Threading.Tasks;
 using Akka.Streams.Dsl;
 using Akka.Streams.TestKit;
 using Akka.Util.Internal;
@@ -32,10 +33,9 @@ protected override TestSubscriber.Probe<int> Setup(IPublisher<int> p1, IPublishe
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_in_the_happy_case()
+        public async Task An_Interleave_for_Flow_must_work_in_the_happy_case()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var probe = this.CreateManualSubscriberProbe<int>();
 
                 Source.From(Enumerable.Range(0, 4))
@@ -52,16 +52,16 @@ public void An_Interleave_for_Flow_must_work_in_the_happy_case()
                     collected.Add(probe.ExpectNext());
                 }
 
-                collected.Should().BeEquivalentTo(new[] {0, 1, 4, 7, 8, 9, 5, 2, 3, 10, 11, 6});
+                collected.Should().BeEquivalentTo(new[] { 0, 1, 4, 7, 8, 9, 5, 2, 3, 10, 11, 6 });
                 probe.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_when_segmentSize_is_not_equal_elements_in_stream()
+        public async Task An_Interleave_for_Flow_must_work_when_segmentSize_is_not_equal_elements_in_stream()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var probe = this.CreateManualSubscriberProbe<int>();
 
                 Source.From(Enumerable.Range(0, 3))
@@ -69,16 +69,16 @@ public void An_Interleave_for_Flow_must_work_when_segmentSize_is_not_equal_eleme
                     .RunWith(Sink.FromSubscriber(probe), Materializer);
 
                 probe.ExpectSubscription().Request(10);
-                probe.ExpectNext( 0, 1, 3, 4, 2, 5);
+                probe.ExpectNext(0, 1, 3, 4, 2, 5);
                 probe.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_with_segmentSize_1()
+        public async Task An_Interleave_for_Flow_must_work_with_segmentSize_1()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var probe = this.CreateManualSubscriberProbe<int>();
 
                 Source.From(Enumerable.Range(0, 3))
@@ -86,27 +86,27 @@ public void An_Interleave_for_Flow_must_work_with_segmentSize_1()
                     .RunWith(Sink.FromSubscriber(probe), Materializer);
 
                 probe.ExpectSubscription().Request(10);
-                probe.ExpectNext( 0, 3, 1, 4, 2, 5);
+                probe.ExpectNext(0, 3, 1, 4, 2, 5);
                 probe.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_not_work_with_segmentSize_0()
+        public async Task An_Interleave_for_Flow_must_not_work_with_segmentSize_0()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var source = Source.From(Enumerable.Range(0, 3));
                 source.Invoking(s => s.Interleave(Source.From(Enumerable.Range(3, 3)), 0))
                     .Should().Throw<ArgumentException>();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_when_segmentSize_is_greater_than_stream_elements()
+        public async Task An_Interleave_for_Flow_must_work_when_segmentSize_is_greater_than_stream_elements()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var probe = this.CreateManualSubscriberProbe<int>();
                 Source.From(Enumerable.Range(0, 3))
                     .Interleave(Source.From(Enumerable.Range(3, 13)), 10)
@@ -126,14 +126,14 @@ public void An_Interleave_for_Flow_must_work_when_segmentSize_is_greater_than_st
                 Enumerable.Range(21, 5).ForEach(i => probe2.ExpectNext(i));
                 Enumerable.Range(11, 10).ForEach(i => probe2.ExpectNext(i));
                 probe2.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_with_one_immediately_completed_and_one_nonempty_publisher()
+        public async Task An_Interleave_for_Flow_must_work_with_one_immediately_completed_and_one_nonempty_publisher()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var subscriber1 = Setup(CompletedPublisher<int>(), NonEmptyPublisher(Enumerable.Range(1, 4)));
                 var subscription1 = subscriber1.ExpectSubscription();
                 subscription1.Request(4);
@@ -146,14 +146,14 @@ public void An_Interleave_for_Flow_must_work_with_one_immediately_completed_and_
                 subscription2.Request(4);
                 Enumerable.Range(1, 4).ForEach(i => subscriber2.ExpectNext(i));
                 subscriber2.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_work_with_one_delayed_completed_and_one_nonempty_publisher()
+        public async Task An_Interleave_for_Flow_must_work_with_one_delayed_completed_and_one_nonempty_publisher()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var subscriber1 = Setup(SoonToCompletePublisher<int>(), NonEmptyPublisher(Enumerable.Range(1, 4)));
                 var subscription1 = subscriber1.ExpectSubscription();
                 subscription1.Request(4);
@@ -166,6 +166,7 @@ public void An_Interleave_for_Flow_must_work_with_one_delayed_completed_and_one_
                 subscription2.Request(4);
                 Enumerable.Range(1, 4).ForEach(i => subscriber2.ExpectNext(i));
                 subscriber2.ExpectComplete();
+                return Task.CompletedTask;
             }, Materializer);
         }
 
@@ -209,10 +210,9 @@ public void An_Interleave_for_Flow_must_work_with_one_delayed_failed_and_one_non
         }
 
         [Fact]
-        public void An_Interleave_for_Flow_must_pass_along_early_cancellation()
+        public async Task An_Interleave_for_Flow_must_pass_along_early_cancellation()
         {
-            this.AssertAllStagesStopped(() =>
-            {
+            await this.AssertAllStagesStoppedAsync(() => {
                 var up1 = this.CreateManualPublisherProbe<int>();
                 var up2 = this.CreateManualPublisherProbe<int>();
                 var down = this.CreateManualSubscriberProbe<int>();
@@ -230,6 +230,7 @@ public void An_Interleave_for_Flow_must_pass_along_early_cancellation()
                 up2.Subscribe(graphSubscriber2);
                 up1.ExpectSubscription().ExpectCancellation();
                 up2.ExpectSubscription().ExpectCancellation();
+                return Task.CompletedTask;
             }, Materializer);
         }
     }