diff --git a/Directory.Packages.props b/Directory.Packages.props
index 2074a81e7f3..250d9b296e4 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -22,7 +22,7 @@
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.8.1" />
     <PackageVersion Include="MinVer" Version="7.0.0" />
     <PackageVersion Include="Newtonsoft.Json" Version="13.0.4" />
-    <PackageVersion Include="NSubstitute" Version="5.3.0" />
+    <PackageVersion Include="NSubstitute" Version="6.0.0" />
     <PackageVersion Include="Polly" Version="8.7.0" />
     <PackageVersion Include="Polly.Contrib.WaitAndRetry" Version="1.1.1" />
     <PackageVersion Include="Polly.Core" Version="8.7.0" />
diff --git a/test/Polly.Core.Tests/Timeout/TimeoutResilienceStrategyTests.cs b/test/Polly.Core.Tests/Timeout/TimeoutResilienceStrategyTests.cs
index eb385122ea1..3e04e4b8570 100644
--- a/test/Polly.Core.Tests/Timeout/TimeoutResilienceStrategyTests.cs
+++ b/test/Polly.Core.Tests/Timeout/TimeoutResilienceStrategyTests.cs
@@ -284,7 +284,7 @@ public async Task Execute_EnsureCancellationTokenRegistrationNotExecutedOnSynchr
         var mockSynchronizationContext = Substitute.For<SynchronizationContext>();
         mockSynchronizationContext
             .When(x => x.Post(Arg.Any<SendOrPostCallback>(), Arg.Any<object>()))
-            .Do((p) => ((SendOrPostCallback)p[1])(p[2]));
+            .Do((p) => ((SendOrPostCallback)p[1]!)(p[2]));
 
         mockSynchronizationContext.CreateCopy()
             .Returns(mockSynchronizationContext);
diff --git a/test/Polly.Extensions.Tests/Registry/ConfigureBuilderContextExtensionsTests.cs b/test/Polly.Extensions.Tests/Registry/ConfigureBuilderContextExtensionsTests.cs
index 711d4bab6fe..efcd8028270 100644
--- a/test/Polly.Extensions.Tests/Registry/ConfigureBuilderContextExtensionsTests.cs
+++ b/test/Polly.Extensions.Tests/Registry/ConfigureBuilderContextExtensionsTests.cs
@@ -27,7 +27,7 @@ public async Task ConfigureReloads_NullArguments_Throws()
     public async Task ConfigureReloads_MonitorRegistrationReturnsNull_DoesNotThrow()
     {
         var monitor = Substitute.For<IOptionsMonitor<Options>>();
-        monitor.OnChange(default!).ReturnsNullForAnyArgs();
+        monitor.OnChange(default!)!.ReturnsNullForAnyArgs();
 
         var listener = new FakeTelemetryListener();
         var registry = new ResiliencePipelineRegistry<string>();