diff --git a/Directory.Packages.props b/Directory.Packages.props
index 8925182..4c87138 100644
--- a/Directory.Packages.props
+++ b/Directory.Packages.props
@@ -16,7 +16,7 @@
     <PackageVersion Include="Microsoft.ML.Tokenizers" Version="2.0.0"/>
     <PackageVersion Include="Microsoft.ML.Tokenizers.Data.Cl100kBase" Version="2.0.0"/>
     <PackageVersion Include="Microsoft.ML.Tokenizers.Data.O200kBase" Version="2.0.0"/>
-    <PackageVersion Include="NSubstitute" Version="5.3.0"/>
+    <PackageVersion Include="NSubstitute" Version="6.0.0"/>
     <PackageVersion Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="10.0.301"/>
     <PackageVersion Include="Microsoft.NET.Test.Sdk" Version="18.7.0"/>
     <PackageVersion Include="Microsoft.VisualStudio.Threading.Analyzers" Version="18.7.23"/>
diff --git a/tests/DotnetTokenKiller.Application.Tests/UseCases/ConfigSetUseCaseTests.cs b/tests/DotnetTokenKiller.Application.Tests/UseCases/ConfigSetUseCaseTests.cs
index b9901e0..847483d 100644
--- a/tests/DotnetTokenKiller.Application.Tests/UseCases/ConfigSetUseCaseTests.cs
+++ b/tests/DotnetTokenKiller.Application.Tests/UseCases/ConfigSetUseCaseTests.cs
@@ -17,7 +17,7 @@ public ConfigSetUseCaseTests()
         _configProvider.LoadAsync().ReturnsForAnyArgs(DtkConfig.Default);
         _configProvider.SaveAsync(null!)
             .ReturnsForAnyArgs(Task.CompletedTask)
-            .AndDoes(call => _savedConfig = call.Arg<DtkConfig>());
+            .AndDoes(call => _savedConfig = call.Arg<DtkConfig>()!);
     }
 
     [Fact]
diff --git a/tests/DotnetTokenKiller.Application.Tests/UseCases/FilteredRunUseCaseTests.cs b/tests/DotnetTokenKiller.Application.Tests/UseCases/FilteredRunUseCaseTests.cs
index b8df469..877d39e 100644
--- a/tests/DotnetTokenKiller.Application.Tests/UseCases/FilteredRunUseCaseTests.cs
+++ b/tests/DotnetTokenKiller.Application.Tests/UseCases/FilteredRunUseCaseTests.cs
@@ -117,7 +117,7 @@ public async Task RunAsync_RecordsCorrectTokenCounts_AfterSuccessfulExecution()
 
         await _tracker.Received(1).RecordAsync(
             Arg.Is<CommandRecord>(r =>
-                r.Command == "build" &&
+                r!.Command == "build" &&
                 r.InputTokens == 6 &&
                 r.OutputTokens == 2 &&
                 r.SavedTokens == 4),
@@ -136,7 +136,7 @@ public async Task RunAsync_RecordsCorrectCommand_WhenArgsProvided()
         await _sut.RunAsync(_filter, "dotnet", BuildArgs, 0);
 
         await _tracker.Received(1).RecordAsync(
-            Arg.Is<CommandRecord>(r => r.Command == "build"),
+            Arg.Is<CommandRecord>(r => r!.Command == "build"),
             Arg.Any<CancellationToken>());
     }
 
@@ -152,7 +152,7 @@ public async Task RunAsync_RecordsCorrectCommand_WhenNoArgs_UsesCommandName()
         await _sut.RunAsync(_filter, "dotnet", [], 0);
 
         await _tracker.Received(1).RecordAsync(
-            Arg.Is<CommandRecord>(r => r.Command == "dotnet"),
+            Arg.Is<CommandRecord>(r => r!.Command == "dotnet"),
             Arg.Any<CancellationToken>());
     }
 
@@ -170,7 +170,7 @@ public async Task RunAsync_RecordsNegativeSavedTokens_WhenFilterExpandsOutput()
 
         await _tracker.Received(1).RecordAsync(
             Arg.Is<CommandRecord>(r =>
-                r.InputTokens == 2 &&
+                r!.InputTokens == 2 &&
                 r.OutputTokens == 6 &&
                 r.SavedTokens == -4 &&
                 r.SavingsPercentage < 0),
@@ -374,7 +374,7 @@ public async Task RunAsync_ZeroTokens_RecordsZeroPercentSavings()
 
         await _tracker.Received(1).RecordAsync(
             Arg.Is<CommandRecord>(r =>
-                r.InputTokens == 0 &&
+                r!.InputTokens == 0 &&
                 r.OutputTokens == 0 &&
                 Math.Abs(r.SavingsPercentage) < 0.01),
             Arg.Any<CancellationToken>());
@@ -395,7 +395,7 @@ public async Task RunAsync_VerifiesSavingsPercentageCalculation()
 
         await _tracker.Received(1).RecordAsync(
             Arg.Is<CommandRecord>(r =>
-                r.InputTokens == 2 &&
+                r!.InputTokens == 2 &&
                 r.OutputTokens == 1 &&
                 r.SavedTokens == 1 &&
                 Math.Abs(r.SavingsPercentage - 50.0) < 0.01),
@@ -499,7 +499,7 @@ public async Task RunAsync_EmptyRawOutput_RecordsZeroSavingsPct()
         await _sut.RunAsync(_filter, "dotnet", BuildArgs, 0);
 
         await _tracker.Received(1).RecordAsync(
-            Arg.Is<CommandRecord>(r => Math.Abs(r.SavingsPercentage) < 0.001),
+            Arg.Is<CommandRecord>(r => Math.Abs(r!.SavingsPercentage) < 0.001),
             Arg.Any<CancellationToken>());
     }
 
@@ -678,7 +678,7 @@ public async Task RunAsync_ZeroExitCode_RecordsSuccessTrue()
         await _sut.RunAsync(_filter, "dotnet", BuildArgs, 0);
 
         await _tracker.Received(1).RecordAsync(
-            Arg.Is<CommandRecord>(r => r.Success),
+            Arg.Is<CommandRecord>(r => r!.Success),
             Arg.Any<CancellationToken>());
     }
 
@@ -695,7 +695,7 @@ public async Task RunAsync_NonZeroExitCode_RecordsSuccessFalse()
         await _sut.RunAsync(_filter, "dotnet", BuildArgs, 0);
 
         await _tracker.Received(1).RecordAsync(
-            Arg.Is<CommandRecord>(r => !r.Success),
+            Arg.Is<CommandRecord>(r => !r!.Success),
             Arg.Any<CancellationToken>());
     }