diff --git a/libavcodec/mpeg12enc.c b/libavcodec/mpeg12enc.c
index 5b8c4ecf33..ea3c7843b9 100644
--- a/libavcodec/mpeg12enc.c
+++ b/libavcodec/mpeg12enc.c
@@ -144,9 +144,6 @@ static av_cold int encode_init(AVCodecContext *avctx)
 {
     MpegEncContext *s = avctx->priv_data;
 
-    if (avctx->codec_id == AV_CODEC_ID_MPEG1VIDEO && avctx->height > 2800)
-        avctx->thread_count = 1;
-
     if (ff_mpv_encode_init(avctx) < 0)
         return -1;
 
diff --git a/libavcodec/pthread_slice.c b/libavcodec/pthread_slice.c
index c8e69f0a9a..f9fc8042fe 100644
--- a/libavcodec/pthread_slice.c
+++ b/libavcodec/pthread_slice.c
@@ -193,6 +193,12 @@ int ff_slice_thread_init(AVCodecContext *avctx)
     w32thread_init();
 #endif
 
+    // We cannot do this in the encoder init as the threads are created before
+    if (av_codec_is_encoder(avctx->codec) &&
+        avctx->codec_id == AV_CODEC_ID_MPEG1VIDEO &&
+        avctx->height > 2800)
+        thread_count = avctx->thread_count = 1;
+
     if (!thread_count) {
         int nb_cpus = av_cpu_count();
         if  (avctx->height)