nunit: Tests not being discovered for .NET 5 target [System.InvalidCastException]

I have a multi-targeted test project that targets .NET Framework 4.6.2 and .NET 5. After building the project in Visual Studio 2022, the test explorer only shows tests for net462 and not for net5.

========== Starting test discovery ========== NUnit Adapter 4.2.0.0: Test discovery starting NUnit Adapter 4.2.0.0: Test discovery complete NUnit Adapter 4.2.0.0: Test discovery starting Exception NUnit.Engine.NUnitEngineException, Exception thrown discovering tests in C:\Work\Workspaces\VisualStudio\dotNET_testing\Builds\net5\Debug\UnitTests.dll The NUnit 3 driver encountered an error while executing reflected code. at NUnit.Engine.Drivers.NUnit3FrameworkDriver.CreateObject(String typeName, Object[] args) at NUnit.Engine.Drivers.NUnit3FrameworkDriver.Load(String testAssemblyPath, IDictionary2 settings) at NUnit.Engine.Runners.DirectTestRunner.LoadDriver(IFrameworkDriver driver, String testFile, TestPackage subPackage) at NUnit.Engine.Runners.DirectTestRunner.LoadPackage() at NUnit.Engine.Runners.TestDomainRunner.LoadPackage() at NUnit.Engine.Runners.DirectTestRunner.EnsurePackageIsLoaded() at NUnit.Engine.Runners.DirectTestRunner.Explore(TestFilter filter) at NUnit.Engine.Runners.MasterTestRunner.Explore(TestFilter filter) at NUnit.VisualStudio.TestAdapter.NUnitEngine.NUnitEngineAdapter.Explore(TestFilter filter) in D:\repos\NUnit\nunit3-vs-adapter\src\NUnitTestAdapter\NUnitEngine\NUnitEngineAdapter.cs:line 88 at NUnit.VisualStudio.TestAdapter.NUnit3TestDiscoverer.DiscoverTests(IEnumerable1 sources, IDiscoveryContext discoveryContext, IMessageLogger messageLogger, ITestCaseDiscoverySink discoverySink) in D:\repos\NUnit\nunit3-vs-adapter\src\NUnitTestAdapter\NUnit3TestDiscoverer.cs:line 82 InnerException: System.InvalidCastException: Unable to cast transparent proxy to type ‘System.Web.UI.ICallbackEventHandler’. at NUnit.Framework.Api.FrameworkController.LoadTestsAction…ctor(FrameworkController controller, Object handler) NUnit Adapter 4.2.0.0: Test discovery complete No test is available in C:\Work\Workspaces\VisualStudio\dotNET_testing\Builds\net5\Debug\UnitTests.dll. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again. ========== Test discovery finished: 248 Tests found in 5.2 sec ==========

Here are the versions of the packages being used:

• Microsoft.NET.Test.Sdk (Version 17.3.0)
• NUnit (Version 3.13.3)
• NUnit.Console (Version 3.15.2)
• NUnit3TestAdapter (Version 4.2.1)