diff --git a/Shared/History.cs b/Shared/History.cs
index 04c6abd..56abd77 100644
--- a/Shared/History.cs
+++ b/Shared/History.cs
@@ -17,7 +17,7 @@ namespace SemiColinGames {
   class History<T> {
 
     // Backing store for the History's items.
-    private T[] items;
+    private readonly T[] items;
     // Points at the most-recently-inserted item.
     private int idx = 0;