diff --git a/src/top/utop.el b/src/top/utop.el
index d7cc985..80a9c8c 100644
--- a/src/top/utop.el
+++ b/src/top/utop.el
@@ -504,7 +504,7 @@ it is started."
       (let ((offsets (split-string argument "," t)))
         (while offsets
           (let ((a (string-to-number (car offsets)))
-                (b (string-to-number (car (cdr offsets)))))
+                (b (string-to-number (cadr offsets))))
             (add-text-properties (+ utop-prompt-max a) (+ utop-prompt-max b) '(face utop-error))
             (setq offsets (cdr (cdr offsets))))))
       ;; Make everything read-only