diff --git a/MediaBrowser.Controller/Net/IAuthService.cs b/MediaBrowser.Controller/Net/IAuthService.cs
index 4c9120e0c9..9132404a08 100644
--- a/MediaBrowser.Controller/Net/IAuthService.cs
+++ b/MediaBrowser.Controller/Net/IAuthService.cs
@@ -1,3 +1,5 @@
+#nullable enable
+
 using MediaBrowser.Controller.Entities;
 using MediaBrowser.Model.Services;
 using Microsoft.AspNetCore.Http;
@@ -7,6 +9,6 @@ namespace MediaBrowser.Controller.Net
     public interface IAuthService
     {
         void Authenticate(IRequest request, IAuthenticationAttributes authAttribtues);
-        User Authenticate(HttpRequest request, IAuthenticationAttributes authAttribtues);
+        User? Authenticate(HttpRequest request, IAuthenticationAttributes authAttribtues);
     }
 }
diff --git a/tests/Jellyfin.Api.Tests/Auth/CustomAuthenticationHandlerTests.cs b/tests/Jellyfin.Api.Tests/Auth/CustomAuthenticationHandlerTests.cs
index a2f5c25016..3b3d03c8b7 100644
--- a/tests/Jellyfin.Api.Tests/Auth/CustomAuthenticationHandlerTests.cs
+++ b/tests/Jellyfin.Api.Tests/Auth/CustomAuthenticationHandlerTests.cs
@@ -83,7 +83,7 @@ namespace Jellyfin.Api.Tests.Auth
                     a => a.Authenticate(
                         It.IsAny<HttpRequest>(),
                         It.IsAny<AuthenticatedAttribute>()))
-                .Returns((User)null);
+                .Returns((User?)null);
 
             var authenticateResult = await _sut.AuthenticateAsync();
 
diff --git a/tests/Jellyfin.Api.Tests/Jellyfin.Api.Tests.csproj b/tests/Jellyfin.Api.Tests/Jellyfin.Api.Tests.csproj
index 0e8ef135ef..1d7e4f7af3 100644
--- a/tests/Jellyfin.Api.Tests/Jellyfin.Api.Tests.csproj
+++ b/tests/Jellyfin.Api.Tests/Jellyfin.Api.Tests.csproj
@@ -3,6 +3,8 @@
   <PropertyGroup>
     <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/tests/Jellyfin.Common.Tests/Jellyfin.Common.Tests.csproj b/tests/Jellyfin.Common.Tests/Jellyfin.Common.Tests.csproj
index da5e6576d3..86bb11bd4c 100644
--- a/tests/Jellyfin.Common.Tests/Jellyfin.Common.Tests.csproj
+++ b/tests/Jellyfin.Common.Tests/Jellyfin.Common.Tests.csproj
@@ -3,6 +3,8 @@
   <PropertyGroup>
     <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/tests/Jellyfin.MediaEncoding.Tests/EncoderValidatorTests.cs b/tests/Jellyfin.MediaEncoding.Tests/EncoderValidatorTests.cs
index a7848316e7..e0f1f236c7 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/EncoderValidatorTests.cs
+++ b/tests/Jellyfin.MediaEncoding.Tests/EncoderValidatorTests.cs
@@ -9,15 +9,15 @@ namespace Jellyfin.MediaEncoding.Tests
 {
     public class EncoderValidatorTests
     {
-        private class GetFFmpegVersionTestData : IEnumerable<object[]>
+        private class GetFFmpegVersionTestData : IEnumerable<object?[]>
         {
-            public IEnumerator<object[]> GetEnumerator()
+            public IEnumerator<object?[]> GetEnumerator()
             {
-                yield return new object[] { EncoderValidatorTestsData.FFmpegV421Output, new Version(4, 2, 1) };
-                yield return new object[] { EncoderValidatorTestsData.FFmpegV42Output, new Version(4, 2) };
-                yield return new object[] { EncoderValidatorTestsData.FFmpegV414Output, new Version(4, 1, 4) };
-                yield return new object[] { EncoderValidatorTestsData.FFmpegV404Output, new Version(4, 0, 4) };
-                yield return new object[] { EncoderValidatorTestsData.FFmpegGitUnknownOutput, null };
+                yield return new object?[] { EncoderValidatorTestsData.FFmpegV421Output, new Version(4, 2, 1) };
+                yield return new object?[] { EncoderValidatorTestsData.FFmpegV42Output, new Version(4, 2) };
+                yield return new object?[] { EncoderValidatorTestsData.FFmpegV414Output, new Version(4, 1, 4) };
+                yield return new object?[] { EncoderValidatorTestsData.FFmpegV404Output, new Version(4, 0, 4) };
+                yield return new object?[] { EncoderValidatorTestsData.FFmpegGitUnknownOutput, null };
             }
 
             IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
@@ -25,7 +25,7 @@ namespace Jellyfin.MediaEncoding.Tests
 
         [Theory]
         [ClassData(typeof(GetFFmpegVersionTestData))]
-        public void GetFFmpegVersionTest(string versionOutput, Version version)
+        public void GetFFmpegVersionTest(string versionOutput, Version? version)
         {
             Assert.Equal(version, EncoderValidator.GetFFmpegVersion(versionOutput));
         }
diff --git a/tests/Jellyfin.MediaEncoding.Tests/Jellyfin.MediaEncoding.Tests.csproj b/tests/Jellyfin.MediaEncoding.Tests/Jellyfin.MediaEncoding.Tests.csproj
index c01edd9fed..b5e4a1287b 100644
--- a/tests/Jellyfin.MediaEncoding.Tests/Jellyfin.MediaEncoding.Tests.csproj
+++ b/tests/Jellyfin.MediaEncoding.Tests/Jellyfin.MediaEncoding.Tests.csproj
@@ -3,6 +3,8 @@
   <PropertyGroup>
     <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/tests/Jellyfin.Naming.Tests/Jellyfin.Naming.Tests.csproj b/tests/Jellyfin.Naming.Tests/Jellyfin.Naming.Tests.csproj
index f246d459ba..9602d9e58a 100644
--- a/tests/Jellyfin.Naming.Tests/Jellyfin.Naming.Tests.csproj
+++ b/tests/Jellyfin.Naming.Tests/Jellyfin.Naming.Tests.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>netcoreapp3.1</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/tests/Jellyfin.Server.Implementations.Tests/Jellyfin.Server.Implementations.Tests.csproj b/tests/Jellyfin.Server.Implementations.Tests/Jellyfin.Server.Implementations.Tests.csproj
index c554bc9372..29733a1c47 100644
--- a/tests/Jellyfin.Server.Implementations.Tests/Jellyfin.Server.Implementations.Tests.csproj
+++ b/tests/Jellyfin.Server.Implementations.Tests/Jellyfin.Server.Implementations.Tests.csproj
@@ -3,6 +3,8 @@
     <PropertyGroup>
       <TargetFramework>netcoreapp3.1</TargetFramework>
       <IsPackable>false</IsPackable>
+      <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+      <Nullable>enable</Nullable>
       <RootNamespace>Jellyfin.Server.Implementations.Tests</RootNamespace>
     </PropertyGroup>