diff --git a/block/commit.c b/block/commit.c
index 444333b..13b55c1 100644
--- a/block/commit.c
+++ b/block/commit.c
@@ -223,6 +223,7 @@ void commit_start(BlockDriverState *bs, BlockDriverState
*base,
BlockDriverState *overlay_bs;
Error *local_err = NULL;
+ assert(cb);
assert(top != bs);
if (top == base) {
error_setg(errp, "Invalid files for merge: top and base are the
same");
diff --git a/block/mirror.c b/block/mirror.c
index a04ed9c..fa2bdab 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -912,6 +912,7 @@ void mirror_start(BlockDriverState *bs, BlockDriverState
*target,
bool is_none_mode;
BlockDriverState *base;
+ assert(cb);
if (mode == MIRROR_SYNC_MODE_INCREMENTAL) {
error_setg(errp, "Sync mode 'incremental' not supported");
return;
@@ -935,6 +936,7 @@ void commit_active_start(BlockDriverState *bs,
BlockDriverState *base,
int ret;
Error *local_err = NULL;
+ assert(cb);
orig_base_flags = bdrv_get_flags(base);
if (bdrv_reopen(base, bs->open_flags, errp)) {
diff --git a/block/stream.c b/block/stream.c
index c0efbda..fc34c63 100644
--- a/block/stream.c
+++ b/block/stream.c
@@ -226,6 +226,7 @@ void stream_start(BlockDriverState *bs, BlockDriverState
*base,
{
StreamBlockJob *s;
+ assert(cb);
s = block_job_create(&stream_job_driver, bs, speed, cb, opaque, errp);
if (!s) {
return;