diff --git a/libavcodec/ac3.h b/libavcodec/ac3.h
index fa290d35e9..f2cb6c39df 100644
--- a/libavcodec/ac3.h
+++ b/libavcodec/ac3.h
@@ -109,7 +109,7 @@ typedef enum AC3PreferredStereoDownmixMode {
     AC3_DMIXMOD_NOTINDICATED = 0,
     AC3_DMIXMOD_LTRT,
     AC3_DMIXMOD_LORO,
-    AC3_DMIXMOD_RESERVED
+    AC3_DMIXMOD_DPLII // reserved value in A/52, but used by encoders to indicate DPL2
 } AC3PreferredStereoDownmixMode;
 
 typedef struct AC3BitAllocParameters {