diff --git a/mps/code/trans.c b/mps/code/trans.c
index a8821d48e04..3fb0d2c304d 100644
--- a/mps/code/trans.c
+++ b/mps/code/trans.c
@@ -170,7 +170,10 @@ Res TransformAddOldNew(Transform transform,
     if (old_list[i] == new_list[i])
       continue;  /* ignore identity-transforms */
 
-    /* Old refs must be in managed memory. */
+    /* .old-white: Old refs must be in managed memory, because
+       transformFix is only reached when a reference is to something
+       in the condemned set.  Other referenes are eliminated by
+       TraceFix, and we can't (currently) transformation of them. */
     {
       Seg seg;
       AVER(SegOfAddr(&seg, transform->arena, old_list[i]));
diff --git a/mps/design/transform.txt b/mps/design/transform.txt
index 96403ce2577..5767283e882 100644
--- a/mps/design/transform.txt
+++ b/mps/design/transform.txt
@@ -92,6 +92,9 @@ Not yet written
 * Why the arena must be parked [When writing this up see
   impl.c.trans.park. RB 2023-06-16].
 
+* Why we can't transform arbitrary references (see
+  impl.c.trans.old-white).
+
 
 References
 ----------
diff --git a/mps/manual/source/topic/transform.rst b/mps/manual/source/topic/transform.rst
index 1d19809aece..9381603436e 100644
--- a/mps/manual/source/topic/transform.rst
+++ b/mps/manual/source/topic/transform.rst
@@ -104,7 +104,9 @@ Interface
 
     ``transform`` is the transform to which the mappings will be added.
 
-    ``old_array`` points to an array of old references.
+    ``old_array`` points to an array of old references, all of which
+    must be to objects in pools whose blocks are automatically managed
+    (see :ref:`pool-properties`).
 
     ``new_array`` points to an array of corresponding new references.