diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 5e0e25d971..7212cb248a 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -4050,7 +4050,7 @@ AVOutputFormat ff_psp_muxer = {
     .write_header      = mov_write_header,
     .write_packet      = mov_write_packet,
     .write_trailer     = mov_write_trailer,
-    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH,
+    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
     .codec_tag         = (const AVCodecTag* const []){ ff_mp4_obj_type, 0 },
     .priv_class        = &psp_muxer_class,
 };
@@ -4103,7 +4103,7 @@ AVOutputFormat ff_ismv_muxer = {
     .write_header      = mov_write_header,
     .write_packet      = mov_write_packet,
     .write_trailer     = mov_write_trailer,
-    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH,
+    .flags             = AVFMT_GLOBALHEADER | AVFMT_ALLOW_FLUSH | AVFMT_TS_NEGATIVE,
     .codec_tag         = (const AVCodecTag* const []){ ff_mp4_obj_type, 0 },
     .priv_class        = &ismv_muxer_class,
 };