diff --git a/extended_report.go b/extended_report.go
index 9bf50e7..57ea47b 100644
--- a/extended_report.go
+++ b/extended_report.go
@@ -562,7 +562,7 @@ func (b *UnknownReportBlock) unpackBlockHeader() {
 
 // MarshalSize returns the size of the packet once marshaled.
 func (x ExtendedReport) MarshalSize() int {
-	return wireSize(x)
+	return headerLength + wireSize(x)
 }
 
 // Marshal encodes the ExtendedReport in binary.
diff --git a/extended_report_test.go b/extended_report_test.go
index ec1cd55..9c975d5 100644
--- a/extended_report_test.go
+++ b/extended_report_test.go
@@ -264,3 +264,22 @@ func TestDecode(t *testing.T) {
 	}
 	assert.True(t, includeSenderSSRC, "DestinationSSRC does not include the SenderSSRC")
 }
+
+func TestExtendedReportMarshalSize(t *testing.T) {
+	packet := testPacket()
+	rawPacket, err := packet.Marshal()
+	assert.NoError(t, err)
+	assert.Equal(t, len(rawPacket), packet.MarshalSize())
+
+	compound := CompoundPacket{
+		&SenderReport{SSRC: 1},
+		&SourceDescription{Chunks: []SourceDescriptionChunk{{
+			Source: 1,
+			Items:  []SourceDescriptionItem{{Type: SDESCNAME, Text: "cname"}},
+		}}},
+		packet,
+	}
+	rawCompound, err := compound.Marshal()
+	assert.NoError(t, err)
+	assert.Equal(t, len(rawCompound), compound.MarshalSize())
+}