diff --git a/lisp/subr.el b/lisp/subr.el
index 4e906bf42c4..c392b28db68 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -5756,7 +5756,7 @@ the function `undo--wrap-and-run-primitive-undo'."
   "Call `primitive-undo' on the undo elements in LIST.
 
 This function is intended to be called purely by `undo' as the
-function in an \(apply DELTA BEG END FUNNAME . ARGS) undo
+function in an \(apply DELTA (BEG . END) FUNNAME . ARGS) undo
 element.  It invokes `before-change-functions' and
 `after-change-functions' once each for the entire region \(BEG
 END) rather than once for each individual change.
@@ -5770,7 +5770,7 @@ Undo elements of this form are generated by the macro
   (let* ((beg-delta (- orig-beg beg))
          (end-delta (- orig-end end))
          (list (cond ((/= beg-delta end-delta)
-                      (error "Not yet implemented"))
+                      (error "Unexpected change of region length"))
                      ((= beg-delta 0)
                       list)
                      (t