diff --git a/libavcodec/ptx.c b/libavcodec/ptx.c
index 527e39acc6..76fff26bb6 100644
--- a/libavcodec/ptx.c
+++ b/libavcodec/ptx.c
@@ -55,10 +55,9 @@ static int ptx_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
 
     buf += offset;
 
-    if ((ret = av_image_check_size(w, h, 0, avctx)) < 0)
+    if ((ret = ff_set_dimensions(avctx, w, h)) < 0)
         return ret;
-    if (w != avctx->width || h != avctx->height)
-        avcodec_set_dimensions(avctx, w, h);
+
     if ((ret = ff_get_buffer(avctx, p, 0)) < 0) {
         av_log(avctx, AV_LOG_ERROR, "get_buffer() failed\n");
         return ret;