diff --git a/libavformat/ftp.c b/libavformat/ftp.c
index dcb588dac2..26aa8ebc0e 100644
--- a/libavformat/ftp.c
+++ b/libavformat/ftp.c
@@ -386,7 +386,7 @@ static int ftp_file_size(FTPContext *s)
     static const int size_codes[] = {213, 0};
 
     snprintf(command, sizeof(command), "SIZE %s\r\n", s->path);
-    if (ftp_send_command(s, command, size_codes, &res) == 213 && res) {
+    if (ftp_send_command(s, command, size_codes, &res) == 213 && res && strlen(res) > 4) {
         s->filesize = strtoll(&res[4], NULL, 10);
     } else {
         s->filesize = -1;