diff --git a/libavcodec/vp8.c b/libavcodec/vp8.c
index fe7aa23491..5bf601a8ef 100644
--- a/libavcodec/vp8.c
+++ b/libavcodec/vp8.c
@@ -2550,6 +2550,8 @@ int vp78_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
     enum AVDiscard skip_thresh;
     VP8Frame *av_uninit(curframe), *prev_frame;
 
+    av_assert0(avctx->pix_fmt == AV_PIX_FMT_YUVA420P || avctx->pix_fmt == AV_PIX_FMT_YUV420P);
+
     if (is_vp7)
         ret = vp7_decode_frame_header(s, avpkt->data, avpkt->size);
     else
diff --git a/libavcodec/webp.c b/libavcodec/webp.c
index 7d23cc7435..b2ae5bcbba 100644
--- a/libavcodec/webp.c
+++ b/libavcodec/webp.c
@@ -1327,9 +1327,8 @@ static int vp8_lossy_decode_frame(AVCodecContext *avctx, AVFrame *p,
     if (!s->initialized) {
         ff_vp8_decode_init(avctx);
         s->initialized = 1;
-        if (s->has_alpha)
-            avctx->pix_fmt = AV_PIX_FMT_YUVA420P;
     }
+    avctx->pix_fmt = s->has_alpha ? AV_PIX_FMT_YUVA420P : AV_PIX_FMT_YUV420P;
     s->lossless = 0;
 
     if (data_size > INT_MAX) {