diff --git a/src/callint.c b/src/callint.c
index a3e4984fd16..8b8cb032095 100644
--- a/src/callint.c
+++ b/src/callint.c
@@ -256,7 +256,8 @@ Optional second arg RECORD-FLAG non-nil
 means unconditionally put this command in the command-history.
 Otherwise, this is done only if an arg is read using the minibuffer.
 Optional third arg KEYS, if given, specifies the sequence of events to
-supply if the command inquires which events were used to invoke it.  */)
+supply if the command inquires which events were used to invoke it.
+If KEYS is omitted or nil, the return value of `this-command-keys' is used.  */)
      (function, record_flag, keys)
      Lisp_Object function, record_flag, keys;
 {