diff --git a/libavformat/hlsenc.c b/libavformat/hlsenc.c
index d5058a1a5e..07aa4ff512 100644
--- a/libavformat/hlsenc.c
+++ b/libavformat/hlsenc.c
@@ -178,7 +178,7 @@ static int hls_write_header(AVFormatContext *s)
     int ret, i;
     char *p;
     const char *pattern = "%d.ts";
-    int basename_size = strlen(s->filename) + strlen(pattern);
+    int basename_size = strlen(s->filename) + strlen(pattern) + 1;
 
     hls->number      = 0;