diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index a580396ebd..fb4297478c 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -2193,6 +2193,7 @@ ztest_replay_write(void *arg1, void *arg2, boolean_t byteswap)
 		 * but not always, because we also want to verify correct
 		 * behavior when the data was not recently read into cache.
 		 */
+		ASSERT(doi.doi_data_block_size);
 		ASSERT0(offset % doi.doi_data_block_size);
 		if (ztest_random(4) != 0) {
 			int prefetch = ztest_random(2) ?
diff --git a/module/zfs/spa_log_spacemap.c b/module/zfs/spa_log_spacemap.c
index 0dfe5b8395..6a27f57595 100644
--- a/module/zfs/spa_log_spacemap.c
+++ b/module/zfs/spa_log_spacemap.c
@@ -690,7 +690,8 @@ spa_estimate_metaslabs_to_flush(spa_t *spa)
 		 * based on the incoming rate until we exceed it.
 		 */
 		if (available_blocks >= 0 && available_txgs >= 0) {
-			uint64_t skip_txgs = MIN(available_txgs + 1,
+			uint64_t skip_txgs = (incoming == 0) ?
+			    available_txgs + 1 : MIN(available_txgs + 1,
 			    (available_blocks / incoming) + 1);
 			available_blocks -= (skip_txgs * incoming);
 			available_txgs -= skip_txgs;