diff --git a/libavformat/utils.c b/libavformat/utils.c
index eaeeb7c165..d838bc86c1 100644
--- a/libavformat/utils.c
+++ b/libavformat/utils.c
@@ -2288,8 +2288,12 @@ static void truncate_ts(AVStream *st, AVPacket *pkt){
 //    if(pkt->dts < 0)
 //        pkt->dts= 0;  //this happens for low_delay=0 and b frames, FIXME, needs further invstigation about what we should do here
 
-    pkt->pts &= pts_mask;
-    pkt->dts &= pts_mask;
+    if( pkt->pts != AV_NOPTS_VALUE ) {
+        pkt->pts &= pts_mask;
+    }
+    if( pkt->dts != AV_NOPTS_VALUE ) {
+        pkt->dts &= pts_mask;
+    }
 }
 
 /**