diff --git a/libavcodec/wmaprodec.c b/libavcodec/wmaprodec.c
index a2ee9b0440..8e1675dc78 100644
--- a/libavcodec/wmaprodec.c
+++ b/libavcodec/wmaprodec.c
@@ -403,7 +403,7 @@ static av_cold int decode_init(WMAProDecodeCtx *s, AVCodecContext *avctx, int nu
         s->decode_flags    = AV_RL16(edata_ptr+14);
         channel_mask       = AV_RL32(edata_ptr+2);
         s->bits_per_sample = AV_RL16(edata_ptr);
-        s->nb_channels     = avctx->ch_layout.nb_channels;
+        s->nb_channels     = channel_mask ? av_popcount(channel_mask) : avctx->ch_layout.nb_channels;
 
         if (s->bits_per_sample > 32 || s->bits_per_sample < 1) {
             avpriv_request_sample(avctx, "bits per sample is %d", s->bits_per_sample);