diff --git a/reference/array/sorting.xml b/reference/array/sorting.xml
index a83d2218be..8fdaa0ae59 100644
--- a/reference/array/sorting.xml
+++ b/reference/array/sorting.xml
@@ -29,6 +29,10 @@
     Note: All of these sort functions act directly on the array
     variable itself, as opposed to returning a new sorted array
    </member>
+   <member>
+    If any of these sort functions evaluates two members as equal then the
+    order is undefined (the sorting is not stable).
+   </member>
   </simplelist>
  </para>
  <para>