diff --git a/tools/ismindex.c b/tools/ismindex.c
index b110420bf0..4fd9a781d0 100644
--- a/tools/ismindex.c
+++ b/tools/ismindex.c
@@ -498,7 +498,7 @@ static int handle_file(struct Tracks *tracks, const char *file, int split,
 {
     AVFormatContext *ctx = NULL;
     int err = 0, i, orig_tracks = tracks->nb_tracks;
-    char *ptr;
+    const char *ptr;
     struct Track *track;
 
     err = avformat_open_input(&ctx, file, NULL, NULL);