diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb767a0..0890f65 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - Remove `QCheck2.TestResult.get_instances` as retaining previous test inputs
   cause memory leaks
+- Make `QCheck2.state.res` immutable, silencing a compilation warning
 
 ## 0.21.3
 
diff --git a/src/core/QCheck2.ml b/src/core/QCheck2.ml
index 1bf96d2..c9a5dd5 100644
--- a/src/core/QCheck2.ml
+++ b/src/core/QCheck2.ml
@@ -1562,7 +1562,7 @@ module Test = struct
     step: 'a step;
     handler : 'a handler;
     rand: RS.t;
-    mutable res: 'a TestResult.t;
+    res: 'a TestResult.t;
     mutable cur_count: int;  (** number of iterations remaining to do *)
     mutable cur_max_gen: int; (** maximum number of generations allowed *)
     mutable cur_max_fail: int; (** maximum number of counter-examples allowed *)