diff --git a/Functional/OptionExtensions.cs b/Functional/OptionExtensions.cs
index 9686a0b..153c7a6 100644
--- a/Functional/OptionExtensions.cs
+++ b/Functional/OptionExtensions.cs
@@ -62,7 +62,7 @@ public static class OptionExtensions
   {
     return isSuccess(value) ? new Right<L, R>(ok()) : new Left<L, R>(err(value));
   }
-  public static IEither<L, R> ToEither<Tvalue, L, R>(this Tvalue value, Tvalue successValue, Func<R> ok, Func<Tvalue, L> err)
+  public static IEither<L, R> ToEither<Tvalue, L, R>(this Tvalue value, Tvalue successValue, Func<R> ok, Func<Tvalue, L> err) where Tvalue : notnull
   {
     return value.ToEither(e => e.Equals(successValue), ok, err);
   }