diff --git a/libavcodec/mpeg4videodec.c b/libavcodec/mpeg4videodec.c
index ac049b95fc..60b06ad750 100644
--- a/libavcodec/mpeg4videodec.c
+++ b/libavcodec/mpeg4videodec.c
@@ -3774,15 +3774,40 @@ int ff_mpeg4_frame_end(AVCodecContext *avctx, const AVPacket *pkt)
 
 #if CONFIG_MPEG4_DECODER
 #if HAVE_THREADS
+static int update_mpvctx(MpegEncContext *s, const MpegEncContext *s1)
+{
+    AVCodecContext *avctx = s->avctx;
+    // FIXME the following leads to a data race; instead copy only
+    // the necessary fields.
+    memcpy(s, s1, sizeof(*s));
+
+    s->context_initialized = 0;
+    s->context_reinit      = 0;
+    s->avctx = avctx;
+
+    if (s1->context_initialized) {
+        int err = ff_mpv_common_init(s);
+        if (err < 0)
+            return err;
+    }
+    return 0;
+}
+
 static int mpeg4_update_thread_context(AVCodecContext *dst,
                                        const AVCodecContext *src)
 {
     Mpeg4DecContext *s = dst->priv_data;
     const Mpeg4DecContext *s1 = src->priv_data;
     int init = s->m.context_initialized;
+    int ret;
 
-    int ret = ff_mpeg_update_thread_context(dst, src);
+    if (!init) {
+        ret = update_mpvctx(&s->m, &s1->m);
+        if (ret < 0)
+            return ret;
+    }
 
+    ret = ff_mpeg_update_thread_context(dst, src);
     if (ret < 0)
         return ret;
 
diff --git a/libavcodec/mpegvideo_dec.c b/libavcodec/mpegvideo_dec.c
index 75c765218c..b8b84ffd8d 100644
--- a/libavcodec/mpegvideo_dec.c
+++ b/libavcodec/mpegvideo_dec.c
@@ -87,25 +87,6 @@ int ff_mpeg_update_thread_context(AVCodecContext *dst,
 
     av_assert0(s != s1);
 
-    // FIXME can parameters change on I-frames?
-    // in that case dst may need a reinit
-    if (!s->context_initialized) {
-        void *private_ctx = s->private_ctx;
-        int err;
-        memcpy(s, s1, sizeof(*s));
-
-        s->context_initialized   = 0;
-        s->context_reinit        = 0;
-        s->avctx                 = dst;
-        s->private_ctx           = private_ctx;
-
-        if (s1->context_initialized) {
-            if ((err = ff_mpv_common_init(s)) < 0)
-                return err;
-            ret = 1;
-        }
-    }
-
     if (s->height != s1->height || s->width != s1->width || s->context_reinit) {
         s->height = s1->height;
         s->width  = s1->width;