diff --git a/module/language/cps/intmap.scm b/module/language/cps/intmap.scm
index 146b55039..9081f3362 100644
--- a/module/language/cps/intmap.scm
+++ b/module/language/cps/intmap.scm
@@ -174,7 +174,10 @@
         (#f (let ((v (new-branch edit)))
               (vector-set! root idx v)
               v))
-        (v (writable-branch v edit)))))
+        (v (let ((v* (writable-branch v edit)))
+             (unless (eq? v v*)
+               (vector-set! root idx v*))
+             v*)))))
   (define (adjoin! i shift root)
     (let* ((shift (- shift *branch-bits*))
            (idx (logand (ash i (- shift)) *branch-mask*)))