diff --git a/library/src/main/java/com/google/android/exoplayer/extractor/webm/WebmExtractor.java b/library/src/main/java/com/google/android/exoplayer/extractor/webm/WebmExtractor.java
index a63ad5d13d..6da0853616 100644
--- a/library/src/main/java/com/google/android/exoplayer/extractor/webm/WebmExtractor.java
+++ b/library/src/main/java/com/google/android/exoplayer/extractor/webm/WebmExtractor.java
@@ -646,6 +646,7 @@ public final class WebmExtractor implements Extractor {
               // Write a start code for the current NAL unit.
               nalStartCode.setPosition(0);
               trackOutput.sampleData(nalStartCode, 4);
+              sampleSize += nalUnitLengthFieldLengthDiff;
             } else {
               // Write the payload of the NAL unit.
               int writtenBytes = trackOutput.sampleData(input, sampleCurrentNalBytesRemaining);