diff --git a/fftools/ffmpeg_opt.c b/fftools/ffmpeg_opt.c
index 48e6816c19..55268cd0a5 100644
--- a/fftools/ffmpeg_opt.c
+++ b/fftools/ffmpeg_opt.c
@@ -645,6 +645,8 @@ static int opt_map(void *optctx, const char *opt, const char *arg)
             for (i = 0; i < o->nb_stream_maps; i++) {
                 m = &o->stream_maps[i];
                 if (file_idx == m->file_index &&
+                    m->stream_index >= 0 &&
+                    m->stream_index < input_files[m->file_index]->nb_streams &&
                     stream_specifier_match(&ss,
                                            input_files[m->file_index]->ctx,
                                            input_files[m->file_index]->ctx->streams[m->stream_index],