diff --git a/libavformat/movenc.c b/libavformat/movenc.c
index 4ed99b7ea3..e2591681a5 100644
--- a/libavformat/movenc.c
+++ b/libavformat/movenc.c
@@ -2869,10 +2869,12 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
         memcpy(trk->vos_data, pkt->data, size);
     }
 
-    if (!(trk->entry % MOV_INDEX_CLUSTER_SIZE)) {
-        trk->cluster = av_realloc(trk->cluster, (trk->entry + MOV_INDEX_CLUSTER_SIZE) * sizeof(*trk->cluster));
-        if (!trk->cluster)
-            return -1;
+    if (trk->entry >= trk->cluster_capacity) {
+        unsigned new_capacity = trk->entry + MOV_INDEX_CLUSTER_SIZE;
+        if (av_reallocp_array(&trk->cluster, new_capacity,
+                              sizeof(*trk->cluster)))
+            return AVERROR(ENOMEM);
+        trk->cluster_capacity = new_capacity;
     }
 
     trk->cluster[trk->entry].pos = avio_tell(pb) - size;
diff --git a/libavformat/movenc.h b/libavformat/movenc.h
index 879c942a9f..2aa8ffb7e4 100644
--- a/libavformat/movenc.h
+++ b/libavformat/movenc.h
@@ -93,6 +93,7 @@ typedef struct MOVTrack {
     int         vos_len;
     uint8_t     *vos_data;
     MOVIentry   *cluster;
+    unsigned    cluster_capacity;
     int         audio_vbr;
     int         height; ///< active picture (w/o VBI) height for D-10/IMX
     uint32_t    tref_tag;