diff --git a/block/vhdx.c b/block/vhdx.c
index bec10bd..46d504c 100644
--- a/block/vhdx.c
+++ b/block/vhdx.c
@@ -1774,7 +1774,7 @@
     log_size = qemu_opt_get_size_del(opts, VHDX_BLOCK_OPT_LOG_SIZE, 0);
     block_size = qemu_opt_get_size_del(opts, VHDX_BLOCK_OPT_BLOCK_SIZE, 0);
     type = qemu_opt_get_del(opts, BLOCK_OPT_SUBFMT);
-    use_zero_blocks = qemu_opt_get_bool_del(opts, VHDX_BLOCK_OPT_ZERO, false);
+    use_zero_blocks = qemu_opt_get_bool_del(opts, VHDX_BLOCK_OPT_ZERO, true);
 
     if (image_size > VHDX_MAX_IMAGE_SIZE) {
         error_setg_errno(errp, EINVAL, "Image size too large; max of 64TB");
@@ -1936,7 +1936,9 @@
        {
            .name = VHDX_BLOCK_OPT_ZERO,
            .type = QEMU_OPT_BOOL,
-           .help = "Force use of payload blocks of type 'ZERO'.  Non-standard."
+           .help = "Force use of payload blocks of type 'ZERO'. "\
+                   "Non-standard, but default.  Do not set to 'off' when "\
+                   "using 'qemu-img convert' with subformat=dynamic."
        },
        { NULL }
     }
diff --git a/qemu-doc.texi b/qemu-doc.texi
index ad418f8..aabe8df 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -698,7 +698,11 @@
 Specifies which VHDX subformat to use. Valid options are
 @code{dynamic} (default) and @code{fixed}.
 @item block_state_zero
-Force use of payload blocks of type 'ZERO'.
+Force use of payload blocks of type 'ZERO'.  Can be set to @code{on} (default)
+or @code{off}.  When set to @code{off}, new blocks will be created as
+@code{PAYLOAD_BLOCK_NOT_PRESENT}, which means parsers are free to return
+arbitrary data for those blocks.  Do not set to @code{off} when using
+@code{qemu-img convert} with @code{subformat=dynamic}.
 @item block_size
 Block size; min 1 MB, max 256 MB.  0 means auto-calculate based on image size.
 @item log_size