diff --git a/libavformat/rtsp.c b/libavformat/rtsp.c
index 6da03d26fe..45b62c4188 100644
--- a/libavformat/rtsp.c
+++ b/libavformat/rtsp.c
@@ -612,7 +612,8 @@ static void sdp_parse_line(AVFormatContext *s, SDPParseState *s1,
                              NULL, NULL, 0, p);
                 if (proto[0] == '\0') {
                     /* relative control URL */
-                    if (rtsp_st->control_url[strlen(rtsp_st->control_url)-1]!='/')
+                    size_t len = strlen(rtsp_st->control_url);
+                    if (len == 0 || rtsp_st->control_url[len - 1] != '/')
                         av_strlcat(rtsp_st->control_url, "/",
                                    sizeof(rtsp_st->control_url));
                     av_strlcat(rtsp_st->control_url, p,