diff --git a/MoreLinq/ReverseComparer.cs b/MoreLinq/ReverseComparer.cs
index 859a1c645..bb46d3da5 100644
--- a/MoreLinq/ReverseComparer.cs
+++ b/MoreLinq/ReverseComparer.cs
@@ -23,7 +23,7 @@ sealed class ReverseComparer<T> : IComparer<T>
     {
         readonly IComparer<T> _underlying;
 
-        public ReverseComparer(IComparer<T> underlying)
+        public ReverseComparer(IComparer<T>? underlying)
         {
             _underlying = underlying ?? Comparer<T>.Default;
         }