diff --git a/CSharp/Clipper2Lib/Clipper.Engine.cs b/CSharp/Clipper2Lib/Clipper.Engine.cs
index 7a2cadc3..b4c28ca4 100644
--- a/CSharp/Clipper2Lib/Clipper.Engine.cs
+++ b/CSharp/Clipper2Lib/Clipper.Engine.cs
@@ -62,7 +62,7 @@ public Vertex(Point64 pt, VertexFlags flags, Vertex? prev)
     }
   };
 
-  internal struct LocalMinima
+  internal readonly struct LocalMinima
   {
     public readonly Vertex vertex;
     public readonly PathType polytype;
@@ -85,7 +85,7 @@ public LocalMinima(Vertex vertex, PathType polytype, bool isOpen = false)
       return !(lm1 == lm2);
     }
 
-    public override bool Equals(object obj)
+    public override bool Equals(object? obj)
     {
       return obj is LocalMinima minima && this == minima;
     }