diff --git a/plugin/cache/cache.go b/plugin/cache/cache.go
index 4a6377b9a..6dc6dc0a2 100644
--- a/plugin/cache/cache.go
+++ b/plugin/cache/cache.go
@@ -182,6 +182,7 @@ func (w *ResponseWriter) RemoteAddr() net.Addr {
 
 // WriteMsg implements the dns.ResponseWriter interface.
 func (w *ResponseWriter) WriteMsg(res *dns.Msg) error {
+	res = res.Copy()
 	mt, _ := response.Typify(res, w.now().UTC())
 
 	// key returns empty string for anything we don't want to cache.