Exception: Unable to cast object of type 'NUnit.Core.NamespaceSuite' to type 'NUnit.Core.TestFixture'

Question

I am getting a typecast exception while executing the below code :

public static void Main(String[] args)
{            
    string testpackagepath = @"C:\Users\pjena\source\repos\Lacerte.Test\Lacerte.Test.Nunit\bin\Debug\Lacerte.Test.Nunit.dll";

    CoreExtensions.Host.InitializeService();
    TestSuiteBuilder builder = new TestSuiteBuilder();
    TestPackage testPackage = new TestPackage(testpackagepath);
    RemoteTestRunner remoteTestRunner = new RemoteTestRunner();
    remoteTestRunner.Load(testPackage);

    TestSuite suite = builder.Build(testPackage);
    TestSuite test = suite.Tests[0] as TestSuite;
    var numberOfTests = ((TestFixture)test.Tests[0]).TestCount;

    foreach (TestMethod t in ((TestFixture)test.Tests[0]).Tests)
    {
        Console.WriteLine(t.TestName.Name);
    }

    TestName testName = ((TestMethod)((TestFixture)test.Tests[0]).Tests[0]).TestName;
    TestFilter filter = new NameFilter(testName);
    TestResult result = test.Run(new NullListener(), filter);
    ResultSummarizer summ = new ResultSummarizer(result);
    Assert.AreEqual(1, summ.ResultCount);
}

Error I am getting:

"Unable to cast object of type 'NUnit.Core.NamespaceSuite' to type 'NUnit.Core.TestFixture'."

I was trying one answer give in this forum and landed into this problem Nunit-C#: run specific Tests through coding