diff --git a/protocol/rtsp/rtsp.go b/protocol/rtsp/rtsp.go
index 7f032967..1ca0cec5 100644
--- a/protocol/rtsp/rtsp.go
+++ b/protocol/rtsp/rtsp.go
@@ -133,7 +133,7 @@ func ReadResponse(r io.Reader) (*Response, error) {
 	}
 	s := scanner.Text()
 
-	if len(s) < minResponse || s[:5] != "RTSP/" {
+	if len(s) < minResponse || !strings.HasPrefix(s, "RTSP/") {
 		return nil, errInvalidResponse
 	}
 	resp.Proto = "RTSP"