diff --git a/clash/format.lisp b/clash/format.lisp
index 949d2f0..7d32329 100644
--- a/clash/format.lisp
+++ b/clash/format.lisp
@@ -192,9 +192,9 @@ parse-format-string)."
       ((eq :index part)
        (format stream "~A" index))
       ((and (consp part) (eq :index (car part)))
-       (case (car part)
-         (:left (format stream "~VD" max-index-length index))
-         (:right (format stream "~V@<~D~>" max-index-length index))
+       (case (cdr part)
+         (:left (format stream "~V@<~D~>" max-index-length index))
+         (:right (format stream "~VD" max-index-length index))
          (t (format stream "~A" index))))
       ((stringp part)
        (format stream "~A" part))