diff --git a/libavcodec/ffv1.c b/libavcodec/ffv1.c
index 6bcdf7c0ad..13d3be2b56 100644
--- a/libavcodec/ffv1.c
+++ b/libavcodec/ffv1.c
@@ -101,7 +101,7 @@ av_cold int ff_ffv1_init_slice_state(FFV1Context *f, FFV1Context *fs)
 av_cold int ff_ffv1_init_slices_state(FFV1Context *f)
 {
     int i, ret;
-    for (i = 0; i < f->slice_count; i++) {
+    for (i = 0; i < f->max_slice_count; i++) {
         FFV1Context *fs = f->slice_context[i];
         if ((ret = ff_ffv1_init_slice_state(f, fs)) < 0)
             return AVERROR(ENOMEM);
@@ -113,10 +113,10 @@ av_cold int ff_ffv1_init_slice_contexts(FFV1Context *f)
 {
     int i;
 
-    f->slice_count = f->num_h_slices * f->num_v_slices;
-    av_assert0(f->slice_count > 0);
+    f->max_slice_count = f->num_h_slices * f->num_v_slices;
+    av_assert0(f->max_slice_count > 0);
 
-    for (i = 0; i < f->slice_count; i++) {
+    for (i = 0; i < f->max_slice_count; i++) {
         int sx          = i % f->num_h_slices;
         int sy          = i / f->num_h_slices;
         int sxs         = f->avctx->width  *  sx      / f->num_h_slices;
@@ -210,7 +210,7 @@ av_cold int ff_ffv1_close(AVCodecContext *avctx)
         ff_thread_release_buffer(avctx, &s->last_picture);
     av_frame_free(&s->last_picture.f);
 
-    for (j = 0; j < s->slice_count; j++) {
+    for (j = 0; j < s->max_slice_count; j++) {
         FFV1Context *fs = s->slice_context[j];
         for (i = 0; i < s->plane_count; i++) {
             PlaneContext *p = &fs->plane[i];
@@ -224,14 +224,14 @@ av_cold int ff_ffv1_close(AVCodecContext *avctx)
     av_freep(&avctx->stats_out);
     for (j = 0; j < s->quant_table_count; j++) {
         av_freep(&s->initial_states[j]);
-        for (i = 0; i < s->slice_count; i++) {
+        for (i = 0; i < s->max_slice_count; i++) {
             FFV1Context *sf = s->slice_context[i];
             av_freep(&sf->rc_stat2[j]);
         }
         av_freep(&s->rc_stat2[j]);
     }
 
-    for (i = 0; i < s->slice_count; i++)
+    for (i = 0; i < s->max_slice_count; i++)
         av_freep(&s->slice_context[i]);
 
     return 0;
diff --git a/libavcodec/ffv1.h b/libavcodec/ffv1.h
index 9526b1634f..b68ed2c4c1 100644
--- a/libavcodec/ffv1.h
+++ b/libavcodec/ffv1.h
@@ -118,6 +118,7 @@ typedef struct FFV1Context {
 
     struct FFV1Context *slice_context[MAX_SLICES];
     int slice_count;
+    int max_slice_count;
     int num_v_slices;
     int num_h_slices;
     int slice_width;
diff --git a/libavcodec/ffv1dec.c b/libavcodec/ffv1dec.c
index 557b1a0994..fbb85d627d 100644
--- a/libavcodec/ffv1dec.c
+++ b/libavcodec/ffv1dec.c
@@ -775,6 +775,7 @@ static int read_header(FFV1Context *f)
             av_log(f->avctx, AV_LOG_ERROR, "read_quant_table error\n");
             return AVERROR_INVALIDDATA;
         }
+        f->slice_count = f->max_slice_count;
     } else if (f->version < 3) {
         f->slice_count = get_symbol(c, state, 0);
     } else {
@@ -789,8 +790,8 @@ static int read_header(FFV1Context *f)
             p -= size + trailer;
         }
     }
-    if (f->slice_count > (unsigned)MAX_SLICES || f->slice_count <= 0) {
-        av_log(f->avctx, AV_LOG_ERROR, "slice count %d is invalid\n", f->slice_count);
+    if (f->slice_count > (unsigned)MAX_SLICES || f->slice_count <= 0 || f->slice_count > f->max_slice_count) {
+        av_log(f->avctx, AV_LOG_ERROR, "slice count %d is invalid (max=%d)\n", f->slice_count, f->max_slice_count);
         return AVERROR_INVALIDDATA;
     }
 
@@ -1016,6 +1017,7 @@ static int init_thread_copy(AVCodecContext *avctx)
     f->picture.f      = NULL;
     f->last_picture.f = NULL;
     f->sample_buffer  = NULL;
+    f->max_slice_count = 0;
     f->slice_count = 0;
 
     for (i = 0; i < f->quant_table_count; i++) {
@@ -1091,7 +1093,7 @@ static int update_thread_context(AVCodecContext *dst, const AVCodecContext *src)
         av_assert0(!fdst->sample_buffer);
     }
 
-    av_assert1(fdst->slice_count == fsrc->slice_count);
+    av_assert1(fdst->max_slice_count == fsrc->max_slice_count);
 
 
     ff_thread_release_buffer(dst, &fdst->picture);
diff --git a/libavcodec/ffv1enc.c b/libavcodec/ffv1enc.c
index 338cc4e3c1..5bd93fb2e9 100644
--- a/libavcodec/ffv1enc.c
+++ b/libavcodec/ffv1enc.c
@@ -982,6 +982,7 @@ slices_ok:
 
     if ((ret = ff_ffv1_init_slice_contexts(s)) < 0)
         return ret;
+    s->slice_count = s->max_slice_count;
     if ((ret = ff_ffv1_init_slices_state(s)) < 0)
         return ret;
 
@@ -991,7 +992,7 @@ slices_ok:
         if (!avctx->stats_out)
             return AVERROR(ENOMEM);
         for (i = 0; i < s->quant_table_count; i++)
-            for (j = 0; j < s->slice_count; j++) {
+            for (j = 0; j < s->max_slice_count; j++) {
                 FFV1Context *sf = s->slice_context[j];
                 av_assert0(!sf->rc_stat2[i]);
                 sf->rc_stat2[i] = av_mallocz(s->context_count[i] *
@@ -1215,6 +1216,7 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
             for (i = 0; i < f->quant_table_count; i++)
                 memset(f->rc_stat2[i], 0, f->context_count[i] * sizeof(*f->rc_stat2[i]));
 
+            av_assert0(f->slice_count == f->max_slice_count);
             for (j = 0; j < f->slice_count; j++) {
                 FFV1Context *fs = f->slice_context[j];
                 for (i = 0; i < 256; i++) {