diff --git a/cmd/ztest/ztest.c b/cmd/ztest/ztest.c
index 3b26f8d1e..0a7ca1c16 100644
--- a/cmd/ztest/ztest.c
+++ b/cmd/ztest/ztest.c
@@ -3715,6 +3715,9 @@ ztest_dmu_read_write(ztest_ds_t *zd, uint64_t id)
 	else
 		dmu_tx_hold_write(tx, bigobj, bigoff, bigsize);
 
+	/* This accounts for setting the checksum/compression. */
+	dmu_tx_hold_bonus(tx, bigobj);
+
 	txg = ztest_tx_assign(tx, TXG_MIGHTWAIT, FTAG);
 	if (txg == 0) {
 		umem_free(packbuf, packsize);
diff --git a/module/zfs/dmu_tx.c b/module/zfs/dmu_tx.c
index 7ee214bb0..5e4a5cbbe 100644
--- a/module/zfs/dmu_tx.c
+++ b/module/zfs/dmu_tx.c
@@ -465,12 +465,12 @@ dmu_tx_count_free(dmu_tx_hold_t *txh, uint64_t off, uint64_t len)
 		blkid = off >> dn->dn_datablkshift;
 		nblks = (len + dn->dn_datablksz - 1) >> dn->dn_datablkshift;
 
-		if (blkid >= dn->dn_maxblkid) {
+		if (blkid > dn->dn_maxblkid) {
 			rw_exit(&dn->dn_struct_rwlock);
 			return;
 		}
 		if (blkid + nblks > dn->dn_maxblkid)
-			nblks = dn->dn_maxblkid - blkid;
+			nblks = dn->dn_maxblkid - blkid + 1;
 
 	}
 	l0span = nblks;    /* save for later use to calc level > 1 overhead */