diff --git a/src/Assert.hack b/src/Assert.hack
index 43fc9fa..073b1c3 100644
--- a/src/Assert.hack
+++ b/src/Assert.hack
@@ -668,7 +668,7 @@ abstract class Assert {
 
   private static function sorted<T>(Traversable<T> $x): ImmVector<T> {
     $copy = Vector::fromItems($x);
-    \sort(&$copy);
+    \sort(inout $copy);
     return $copy->toImmVector();
   }
 }