diff --git a/Utils.ts b/Utils.ts
index 6fae645c5..febcaa3c8 100644
--- a/Utils.ts
+++ b/Utils.ts
@@ -459,6 +459,9 @@ In the case that MapComplete is pointed to the testing grounds, the edit will be
     /**
      * Copies all key-value pairs of the source into the target. This will change the target
      * If the key starts with a '+', the values of the list will be appended to the target instead of overwritten
+     * If the key starts with `=`, the property will be overwritten.
+     *
+     * 'Source' will not be modified, but 'Target' will be
      *
      * const obj = {someValue: 42};
      * const override = {someValue: null};
@@ -494,7 +497,7 @@ In the case that MapComplete is pointed to the testing grounds, the edit will be
      * result.list2.length // =>  1
      * result.list2[0] // => "should-be-untouched"
      */
-    static Merge<T, S>(source: S, target: T): T & S {
+    static Merge<T, S>(source: Readonly<S>, target: T): T & S {
         if (target === null) {
             return <T & S>source
         }