diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
index a0845e4f18..fbacedfdf3 100644
--- a/libavcodec/vp3.c
+++ b/libavcodec/vp3.c
@@ -2321,7 +2321,8 @@ static int theora_decode_header(AVCodecContext *avctx, GetBitContext *gb)
             return AVERROR_INVALIDDATA;
         }
         skip_bits(gb, 3); /* reserved */
-    }
+    } else
+        avctx->pix_fmt = AV_PIX_FMT_YUV420P;
 
     ret = ff_set_dimensions(avctx, s->width, s->height);
     if (ret < 0)