diff --git a/libavformat/subfile.c b/libavformat/subfile.c
index 68d846ab15..e6712806a9 100644
--- a/libavformat/subfile.c
+++ b/libavformat/subfile.c
@@ -137,6 +137,8 @@ static int64_t subfile_seek(URLContext *h, int64_t pos, int whence)
     case SEEK_END:
         new_pos = end + pos;
         break;
+    default:
+        av_assert0(0);
     }
     if (new_pos < c->start)
         return AVERROR(EINVAL);