diff --git a/openrtb2/audio.go b/openrtb2/audio.go
index 1ede214..7eaf906 100644
--- a/openrtb2/audio.go
+++ b/openrtb2/audio.go
@@ -254,7 +254,7 @@ type Audio struct {
 	// Description:
 	//   Volume normalization mode. Refer to List: Volume
 	//   Normalization Modes in AdCOM 1.0.
-	NVol adcom1.VolumeNormalizationMode `json:"nvol,omitempty"`
+	NVol *adcom1.VolumeNormalizationMode `json:"nvol,omitempty"`
 
 	// Attribute:
 	//   ext