diff --git a/src/core/CCVector.ml b/src/core/CCVector.ml
index 03352fdb..62437e2d 100644
--- a/src/core/CCVector.ml
+++ b/src/core/CCVector.ml
@@ -132,9 +132,11 @@ let resize_with v f size =
     v.size <- size
   ) else (
     ensure_assuming_not_empty_ v size;
-    for i = v.size to size - 1 do
-      Array.unsafe_set v.vec i (f i)
+    let {size=cur_size; vec} = v in
+    for i = cur_size to size - 1 do
+      Array.unsafe_set vec i (f i)
     done;
+    assert (size <= Array.length v.vec);
     v.size <- size
   )
 
@@ -146,6 +148,7 @@ let resize_with_init v ~init size =
     v.size <- size
   ) else (
     ensure_assuming_not_empty_ v size;
+    (* nothing will change [v] *)
     for i = v.size to size - 1 do
       Array.unsafe_set v.vec i init
     done;