diff --git a/libavcodec/vp3.c b/libavcodec/vp3.c
index 7df406ca2e..869b6a5f7a 100644
--- a/libavcodec/vp3.c
+++ b/libavcodec/vp3.c
@@ -2299,7 +2299,8 @@ 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;
 
 //    align_get_bits(gb);