diff --git a/MC6809/MC6809.UnitTest/BhiTests.cs b/MC6809/MC6809.UnitTest/BhiTests.cs
index e8126df..d8fd938 100644
--- a/MC6809/MC6809.UnitTest/BhiTests.cs
+++ b/MC6809/MC6809.UnitTest/BhiTests.cs
@@ -6,7 +6,7 @@
     public class BhiTests
     {
         private readonly Board board = new Board();
-        private MC6809 cpu;
+        private readonly MC6809 cpu;
 
         public BhiTests()
         {
diff --git a/MC6809/MC6809.UnitTest/BleTests.cs b/MC6809/MC6809.UnitTest/BleTests.cs
index f028ba0..79ba3e1 100644
--- a/MC6809/MC6809.UnitTest/BleTests.cs
+++ b/MC6809/MC6809.UnitTest/BleTests.cs
@@ -6,7 +6,7 @@
     public class BleTests
     {
         private readonly Board board = new Board();
-        private MC6809 cpu;
+        private readonly MC6809 cpu;
 
         public BleTests()
         {