diff --git a/Z80/Disassembler.cs b/Z80/Disassembler.cs
index 8ae9e86..71e1a5e 100644
--- a/Z80/Disassembler.cs
+++ b/Z80/Disassembler.cs
@@ -104,6 +104,8 @@ namespace Z80
 
         public string Disassemble(Z80 cpu, ushort pc)
         {
+            ArgumentNullException.ThrowIfNull(cpu);
+
             var opCode = this.Bus.Peek(pc);
 
             var decoded = cpu.GetDecodedOpCode(opCode);
diff --git a/Z80/Z80.FuseTest/RegisterState.cs b/Z80/Z80.FuseTest/RegisterState.cs
index 97693ae..1b38255 100644
--- a/Z80/Z80.FuseTest/RegisterState.cs
+++ b/Z80/Z80.FuseTest/RegisterState.cs
@@ -7,7 +7,7 @@ namespace Z80.FuseTest
     using Fuse;
     using System.Globalization;
 
-    internal class RegisterState : AbstractRegisterState, IRegisterState
+    internal sealed class RegisterState : AbstractRegisterState, IRegisterState
     {
         public int I { get; private set; } = -1;
 
diff --git a/Z80/Z80.FuseTest/TestRunner.cs b/Z80/Z80.FuseTest/TestRunner.cs
index 63585fc..62588f7 100644
--- a/Z80/Z80.FuseTest/TestRunner.cs
+++ b/Z80/Z80.FuseTest/TestRunner.cs
@@ -25,7 +25,7 @@ namespace Z80.FuseTest
         MEMPTR,
     }
 
-    internal class TestRunner : EightBit.Bus
+    internal sealed class TestRunner : EightBit.Bus
     {
         private readonly Test<RegisterState> test;
         private readonly Result<RegisterState> result;
diff --git a/Z80/Z80.FuseTest/TestSuite.cs b/Z80/Z80.FuseTest/TestSuite.cs
index 8e61e1e..db3d00e 100644
--- a/Z80/Z80.FuseTest/TestSuite.cs
+++ b/Z80/Z80.FuseTest/TestSuite.cs
@@ -6,7 +6,7 @@ namespace Z80.FuseTest
 {
     using Fuse;
 
-    internal class TestSuite(string path)
+    internal sealed class TestSuite(string path)
     {
         private readonly Tests<RegisterState> tests = new(path + ".in");
         private readonly Results<RegisterState> results = new(path + ".expected");
diff --git a/Z80/Z80.Test/Board.cs b/Z80/Z80.Test/Board.cs
index 2a3289b..dde66bd 100644
--- a/Z80/Z80.Test/Board.cs
+++ b/Z80/Z80.Test/Board.cs
@@ -7,7 +7,7 @@ namespace Z80.Test
     using EightBit;
     using System.Globalization;
 
-    internal class Board : Bus
+    internal sealed class Board : Bus
     {
         private readonly Configuration configuration;
         private readonly Ram ram;