diff --git a/libavcodec/h264.c b/libavcodec/h264.c
index 33a66beb1a..283e753a60 100644
--- a/libavcodec/h264.c
+++ b/libavcodec/h264.c
@@ -1548,8 +1548,14 @@ again:
                     ret = -1;
                     goto end;
                 }
-                if(!idr_cleared)
+                if(!idr_cleared) {
+                    if (h->current_slice && (avctx->active_thread_type & FF_THREAD_SLICE)) {
+                        av_log(h, AV_LOG_ERROR, "invalid mixed IDR / non IDR frames cannot be decoded in slice multithreading mode\n");
+                        ret = AVERROR_INVALIDDATA;
+                        goto end;
+                    }
                     idr(h); // FIXME ensure we don't lose some frames if there is reordering
+                }
                 idr_cleared = 1;
                 h->has_recovery_point = 1;
             case NAL_SLICE: