diff --git a/lisp/xt-mouse.el b/lisp/xt-mouse.el
index 5fb977d99e5..d2d0cf5ee06 100644
--- a/lisp/xt-mouse.el
+++ b/lisp/xt-mouse.el
@@ -70,7 +70,11 @@ http://invisible-island.net/xterm/ctlseqs/ctlseqs.html)."
       (cond
        ((null event) nil)		;Unknown/bogus byte sequence!
        (is-down
-	(setf (terminal-parameter nil 'xterm-mouse-last-down) event)
+	(setf (terminal-parameter nil 'xterm-mouse-last-down)
+              ;; EVENT might be handed back to the input queue, which
+              ;; might modify it.  Copy it into the terminal parameter
+              ;; to guard against that.
+              (copy-sequence event))
 	vec)
        (is-move vec)
        (t