diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 8a5690678d..8d59bd7288 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -1341,7 +1341,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk)
     for (ch = 1; ch <= s->channels; ch++) {
         int audio_channel = 0;
         INTFLOAT gain;
-        if (s->channel_mode == AC3_CHMODE_DUALMONO)
+        if (s->channel_mode == AC3_CHMODE_DUALMONO && ch <= 2)
             audio_channel = 2-ch;
         if (s->heavy_compression && s->compression_exists[audio_channel])
             gain = s->heavy_dynamic_range[audio_channel];