metainfo/announcelist.go | 7 +++++++
 torrent.go               | 2 +-

diff --git a/metainfo/announcelist.go b/metainfo/announcelist.go
index 3a5c09d48349f896134b4ba7e0cb7429d3553853..b371f70d4e31d7fd23882fff3e2bd047d5db0f01 100644
--- a/metainfo/announcelist.go
+++ b/metainfo/announcelist.go
@@ -2,6 +2,13 @@ package metainfo
 
 type AnnounceList [][]string
 
+func (al AnnounceList) Clone() (ret AnnounceList) {
+	for _, tier := range al {
+		ret = append(ret, append([]string(nil), tier...))
+	}
+	return
+}
+
 // Whether the AnnounceList should be preferred over a single URL announce.
 func (al AnnounceList) OverridesAnnounce(announce string) bool {
 	for _, tier := range al {
diff --git a/torrent.go b/torrent.go
index db0126bfac1ccd8b3f03d23b463924dca41dca20..d29c9a923525eff29ac18a0d15f699fcc8550a07 100644
--- a/torrent.go
+++ b/torrent.go
@@ -633,7 +633,7 @@ 	return metainfo.MetaInfo{
 		CreationDate: time.Now().Unix(),
 		Comment:      "dynamic metainfo from client",
 		CreatedBy:    "go.torrent",
-		AnnounceList: t.metainfo.UpvertedAnnounceList(),
+		AnnounceList: t.metainfo.UpvertedAnnounceList().Clone(),
 		InfoBytes: func() []byte {
 			if t.haveInfo() {
 				return t.metadataBytes