diff --git a/src/main/java/com/netflix/imflibrary/writerTools/IMPAssembler.java b/src/main/java/com/netflix/imflibrary/writerTools/IMPAssembler.java
index 88ffdd03..3b97b0b8 100644
--- a/src/main/java/com/netflix/imflibrary/writerTools/IMPAssembler.java
+++ b/src/main/java/com/netflix/imflibrary/writerTools/IMPAssembler.java
@@ -61,7 +61,7 @@ public AssembledIMPResult assembleIMFFromFiles(SimpleTimeline simpleTimeline, Fi
         Map<UUID, List<Long>> sampleRateMap = new HashMap<>();
         Map<UUID, BigInteger> sampleCountMap = new HashMap<>();
         Map<UUID, byte[]> hashMap = new HashMap<>();
-        long videoIntrinsicDuration = 0;
+        long videoTotalSourceDuration = 0;
 
 
         for (Track track : simpleTimeline.getEssenceTracks()) {
@@ -159,7 +159,7 @@ public AssembledIMPResult assembleIMFFromFiles(SimpleTimeline simpleTimeline, Fi
                     logger.info("Adding file to resources: {}..", essenceTrackEntry.getFile().getName());
 
                     if (track.getSequenceTypeEnum().equals(Composition.SequenceTypeEnum.MainImageSequence)) {
-                        videoIntrinsicDuration += sampleCount.longValue();
+                        videoTotalSourceDuration += (essenceTrackEntry.getDuration() == null ? sampleCount : essenceTrackEntry.getDuration()).longValue();
                     }
 
                     trackFileResources.add(
@@ -209,9 +209,9 @@ public AssembledIMPResult assembleIMFFromFiles(SimpleTimeline simpleTimeline, Fi
             markerResources.add(new IMFMarkerResourceType(
                     UUIDHelper.fromUUID(IMFUUIDGenerator.getInstance().generateUUID()),
                     editRate,
-                    BigInteger.valueOf(videoIntrinsicDuration),
+                    BigInteger.valueOf(videoTotalSourceDuration),
                     BigInteger.ZERO,
-                    BigInteger.valueOf(videoIntrinsicDuration), // source duration may not be necessary
+                    BigInteger.valueOf(videoTotalSourceDuration), // source duration may not be necessary
                     BigInteger.ONE,
                     markerList));