diff --git a/block/io.c b/block/io.c
index 33150c03590..395bea3bace 100644
--- a/block/io.c
+++ b/block/io.c
@@ -1726,22 +1726,29 @@ static int bdrv_pad_request(BlockDriverState *bs,
         return 0;
     }
 
-    sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
-                                  &sliced_head, &sliced_tail,
-                                  &sliced_niov);
-
-    /* Guaranteed by bdrv_check_request32() */
-    assert(*bytes <= SIZE_MAX);
-    ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
-                                  sliced_head, *bytes);
-    if (ret < 0) {
-        bdrv_padding_finalize(pad);
-        return ret;
+    /*
+     * For prefetching in stream_populate(), no qiov is passed along, because
+     * only copy-on-read matters.
+     */
+    if (qiov && *qiov) {
+        sliced_iov = qemu_iovec_slice(*qiov, *qiov_offset, *bytes,
+                                      &sliced_head, &sliced_tail,
+                                      &sliced_niov);
+
+        /* Guaranteed by bdrv_check_request32() */
+        assert(*bytes <= SIZE_MAX);
+        ret = bdrv_create_padded_qiov(bs, pad, sliced_iov, sliced_niov,
+                                      sliced_head, *bytes);
+        if (ret < 0) {
+            bdrv_padding_finalize(pad);
+            return ret;
+        }
+        *qiov = &pad->local_qiov;
+        *qiov_offset = 0;
     }
+
     *bytes += pad->head + pad->tail;
     *offset -= pad->head;
-    *qiov = &pad->local_qiov;
-    *qiov_offset = 0;
     if (padded) {
         *padded = true;
     }