diff --git a/libavformat/dashdec.c b/libavformat/dashdec.c
index c28bb07f44..693fc7372b 100644
--- a/libavformat/dashdec.c
+++ b/libavformat/dashdec.c
@@ -771,7 +771,7 @@ static int resolve_content_path(AVFormatContext *s, const char *url, int *max_ur
     size = strlen(root_url);
     isRootHttp = ishttp(root_url);
 
-    if (root_url[size - 1] != token) {
+    if (size > 0 && root_url[size - 1] != token) {
         av_strlcat(root_url, "/", size + 2);
         size += 2;
     }