diff --git a/src/CommandLine/CastExtensions.cs b/src/CommandLine/CastExtensions.cs
index 828a18e4..fa34928c 100644
--- a/src/CommandLine/CastExtensions.cs
+++ b/src/CommandLine/CastExtensions.cs
@@ -4,7 +4,7 @@
 
 namespace CommandLine
 {
-    public static class CastExtensions
+    internal static class CastExtensions
     {
         private const string ImplicitCastMethodName = "op_Implicit";
         private const string ExplicitCastMethodName = "op_Explicit";