diff --git a/doc/muxers.texi b/doc/muxers.texi
index 33f1f7bbaf..268c152023 100644
--- a/doc/muxers.texi
+++ b/doc/muxers.texi
@@ -1344,6 +1344,8 @@ be negative. This enables the initial sample to have DTS/CTS of zero, and
 reduces the need for edit lists for some cases such as video tracks with
 B-frames. Additionally, eases conformance with the DASH-IF interoperability
 guidelines.
+
+This option is implicitly set when writing ismv (Smooth Streaming) files.
 @item -write_prft
 Write producer time reference box (PRFT) with a specified time source for the
 NTP field in the PRFT box. Set value as @samp{wallclock} to specify timesource
diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index dd6aa02525..72a6e12d52 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -6046,7 +6046,7 @@ static int mov_init(AVFormatContext *s)
     /* Set other implicit flags immediately */
     if (mov->mode == MODE_ISM)
         mov->flags |= FF_MOV_FLAG_EMPTY_MOOV | FF_MOV_FLAG_SEPARATE_MOOF |
-                      FF_MOV_FLAG_FRAGMENT;
+                      FF_MOV_FLAG_FRAGMENT | FF_MOV_FLAG_NEGATIVE_CTS_OFFSETS;
     if (mov->flags & FF_MOV_FLAG_DASH)
         mov->flags |= FF_MOV_FLAG_FRAGMENT | FF_MOV_FLAG_EMPTY_MOOV |
                       FF_MOV_FLAG_DEFAULT_BASE_MOOF;
diff --git a/tests/ref/fate/movenc b/tests/ref/fate/movenc
index 872796ebce..5e8f324ea3 100644
--- a/tests/ref/fate/movenc
+++ b/tests/ref/fate/movenc
@@ -17,10 +17,10 @@ write_data len 20, time nopts, type header atom ftyp
 write_data len 1171, time nopts, type header atom -
 write_data len 728, time 0, type sync atom moof
 write_data len 828, time nopts, type unknown atom -
-write_data len 728, time 1013106, type sync atom moof
+write_data len 728, time 1046439, type sync atom moof
 write_data len 812, time nopts, type unknown atom -
 write_data len 148, time nopts, type trailer atom -
-1f37c1a8e01651e8bebcd66f00b6a226 4435 ismv
+49bf122c4c732a344ef68b58acd19be5 4435 ismv
 write_data len 36, time nopts, type header atom ftyp
 write_data len 1123, time nopts, type header atom -
 write_data len 796, time 0, type sync atom moof
diff --git a/tests/ref/lavf/ismv b/tests/ref/lavf/ismv
index f29b5ff3ef..7b4a466096 100644
--- a/tests/ref/lavf/ismv
+++ b/tests/ref/lavf/ismv
@@ -1,9 +1,9 @@
-a9ccbb4cd1436d222ef4425567b4e03d *./tests/data/lavf/lavf.ismv
+96053075a3f60d271131fe2d0765c267 *./tests/data/lavf/lavf.ismv
 312542 ./tests/data/lavf/lavf.ismv
 ./tests/data/lavf/lavf.ismv CRC=0x9d9a638a
-440d85f9fd5b9f63c2676638782b5c15 *./tests/data/lavf/lavf.ismv
+7022701b4c693bc4ffe1e9f96dd82a02 *./tests/data/lavf/lavf.ismv
 321448 ./tests/data/lavf/lavf.ismv
 ./tests/data/lavf/lavf.ismv CRC=0xe8130120
-a9ccbb4cd1436d222ef4425567b4e03d *./tests/data/lavf/lavf.ismv
+96053075a3f60d271131fe2d0765c267 *./tests/data/lavf/lavf.ismv
 312542 ./tests/data/lavf/lavf.ismv
 ./tests/data/lavf/lavf.ismv CRC=0x9d9a638a