diff --git a/msg.go b/msg.go
index 8595d12..7be9ede 100644
--- a/msg.go
+++ b/msg.go
@@ -326,7 +326,7 @@ func zipMessages() ([]byte, error) {
 func UnzipMessages(z []byte) error {
 	log.Printf("UnzipMessages : %d bytes.\n", len(z))
 	r := bytes.NewReader(z)
-	zr, err := zip.NewReader(r, len(z))
+	zr, err := zip.NewReader(r, int64(len(z)))
 
 	for _, f := range zr.File {
 		log.Printf("File : %s\n", f.Name)