diff --git a/include/sys/dmu_tx.h b/include/sys/dmu_tx.h
index 7d2fdab290..1ee513fdc8 100644
--- a/include/sys/dmu_tx.h
+++ b/include/sys/dmu_tx.h
@@ -171,7 +171,7 @@ extern dmu_tx_t *dmu_tx_create_assigned(struct dsl_pool *dp, uint64_t txg);
 dmu_tx_t *dmu_tx_create_dd(dsl_dir_t *dd);
 int dmu_tx_is_syncing(dmu_tx_t *tx);
 int dmu_tx_private_ok(dmu_tx_t *tx);
-void dmu_tx_add_new_object(dmu_tx_t *tx, objset_t *os, dnode_t *dn);
+void dmu_tx_add_new_object(dmu_tx_t *tx, dnode_t *dn);
 void dmu_tx_willuse_space(dmu_tx_t *tx, int64_t delta);
 void dmu_tx_dirty_buf(dmu_tx_t *tx, struct dmu_buf_impl *db);
 int dmu_tx_holds(dmu_tx_t *tx, uint64_t object);
diff --git a/module/zfs/dmu_object.c b/module/zfs/dmu_object.c
index 3e3efe0c13..8ca699ebff 100644
--- a/module/zfs/dmu_object.c
+++ b/module/zfs/dmu_object.c
@@ -131,7 +131,7 @@ dmu_object_alloc_dnsize(objset_t *os, dmu_object_type_t ot, int blocksize,
 	dnode_allocate(dn, ot, blocksize, 0, bonustype, bonuslen, dn_slots, tx);
 	mutex_exit(&os->os_obj_lock);
 
-	dmu_tx_add_new_object(tx, os, dn);
+	dmu_tx_add_new_object(tx, dn);
 	dnode_rele(dn, FTAG);
 
 	return (object);
@@ -168,7 +168,7 @@ dmu_object_claim_dnsize(objset_t *os, uint64_t object, dmu_object_type_t ot,
 		return (err);
 
 	dnode_allocate(dn, ot, blocksize, 0, bonustype, bonuslen, dn_slots, tx);
-	dmu_tx_add_new_object(tx, os, dn);
+	dmu_tx_add_new_object(tx, dn);
 
 	dnode_rele(dn, FTAG);
 
diff --git a/module/zfs/dmu_tx.c b/module/zfs/dmu_tx.c
index 7c1801be47..4d4c74f512 100644
--- a/module/zfs/dmu_tx.c
+++ b/module/zfs/dmu_tx.c
@@ -119,7 +119,7 @@ dmu_tx_hold_dnode_impl(dmu_tx_t *tx, dnode_t *dn, enum dmu_tx_hold_type type,
 	dmu_tx_hold_t *txh;
 
 	if (dn != NULL) {
-		refcount_add(&dn->dn_holds, tx);
+		(void) refcount_add(&dn->dn_holds, tx);
 		if (tx->tx_txg != 0) {
 			mutex_enter(&dn->dn_mtx);
 			/*
@@ -163,7 +163,7 @@ dmu_tx_hold_object_impl(dmu_tx_t *tx, objset_t *os, uint64_t object,
 
 	if (object != DMU_NEW_OBJECT) {
 		err = dnode_hold(os, object, FTAG, &dn);
-		if (err) {
+		if (err != 0) {
 			tx->tx_err = err;
 			return (NULL);
 		}
@@ -175,7 +175,7 @@ dmu_tx_hold_object_impl(dmu_tx_t *tx, objset_t *os, uint64_t object,
 }
 
 void
-dmu_tx_add_new_object(dmu_tx_t *tx, objset_t *os, dnode_t *dn)
+dmu_tx_add_new_object(dmu_tx_t *tx, dnode_t *dn)
 {
 	/*
 	 * If we're syncing, they can manipulate any object anyhow, and
@@ -462,17 +462,16 @@ dmu_tx_hold_write(dmu_tx_t *tx, uint64_t object, uint64_t off, int len)
 {
 	dmu_tx_hold_t *txh;
 
-	ASSERT(tx->tx_txg == 0);
-	ASSERT(len <= DMU_MAX_ACCESS);
+	ASSERT0(tx->tx_txg);
+	ASSERT3U(len, <=, DMU_MAX_ACCESS);
 	ASSERT(len == 0 || UINT64_MAX - off >= len - 1);
 
 	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
 	    object, THT_WRITE, off, len);
-	if (txh == NULL)
-		return;
-
-	dmu_tx_count_write(txh, off, len);
-	dmu_tx_count_dnode(txh);
+	if (txh != NULL) {
+		dmu_tx_count_write(txh, off, len);
+		dmu_tx_count_dnode(txh);
+	}
 }
 
 void
@@ -480,16 +479,15 @@ dmu_tx_hold_write_by_dnode(dmu_tx_t *tx, dnode_t *dn, uint64_t off, int len)
 {
 	dmu_tx_hold_t *txh;
 
-	ASSERT(tx->tx_txg == 0);
-	ASSERT(len <= DMU_MAX_ACCESS);
+	ASSERT0(tx->tx_txg);
+	ASSERT3U(len, <=, DMU_MAX_ACCESS);
 	ASSERT(len == 0 || UINT64_MAX - off >= len - 1);
 
 	txh = dmu_tx_hold_dnode_impl(tx, dn, THT_WRITE, off, len);
-	if (txh == NULL)
-		return;
-
-	dmu_tx_count_write(txh, off, len);
-	dmu_tx_count_dnode(txh);
+	if (txh != NULL) {
+		dmu_tx_count_write(txh, off, len);
+		dmu_tx_count_dnode(txh);
+	}
 }
 
 static void
@@ -792,9 +790,8 @@ dmu_tx_hold_free(dmu_tx_t *tx, uint64_t object, uint64_t off, uint64_t len)
 
 	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
 	    object, THT_FREE, off, len);
-	if (txh == NULL)
-		return;
-	(void) dmu_tx_hold_free_impl(txh, off, len);
+	if (txh != NULL)
+		(void) dmu_tx_hold_free_impl(txh, off, len);
 }
 
 void
@@ -803,9 +800,8 @@ dmu_tx_hold_free_by_dnode(dmu_tx_t *tx, dnode_t *dn, uint64_t off, uint64_t len)
 	dmu_tx_hold_t *txh;
 
 	txh = dmu_tx_hold_dnode_impl(tx, dn, THT_FREE, off, len);
-	if (txh == NULL)
-		return;
-	(void) dmu_tx_hold_free_impl(txh, off, len);
+	if (txh != NULL)
+		(void) dmu_tx_hold_free_impl(txh, off, len);
 }
 
 static void
@@ -916,13 +912,12 @@ dmu_tx_hold_zap(dmu_tx_t *tx, uint64_t object, int add, const char *name)
 {
 	dmu_tx_hold_t *txh;
 
-	ASSERT(tx->tx_txg == 0);
+	ASSERT0(tx->tx_txg);
 
 	txh = dmu_tx_hold_object_impl(tx, tx->tx_objset,
 	    object, THT_ZAP, add, (uintptr_t)name);
-	if (txh == NULL)
-		return;
-	dmu_tx_hold_zap_impl(txh, add, name);
+	if (txh != NULL)
+		dmu_tx_hold_zap_impl(txh, add, name);
 }
 
 void
@@ -930,13 +925,12 @@ dmu_tx_hold_zap_by_dnode(dmu_tx_t *tx, dnode_t *dn, int add, const char *name)
 {
 	dmu_tx_hold_t *txh;
 
-	ASSERT(tx->tx_txg == 0);
+	ASSERT0(tx->tx_txg);
 	ASSERT(dn != NULL);
 
 	txh = dmu_tx_hold_dnode_impl(tx, dn, THT_ZAP, add, (uintptr_t)name);
-	if (txh == NULL)
-		return;
-	dmu_tx_hold_zap_impl(txh, add, name);
+	if (txh != NULL)
+		dmu_tx_hold_zap_impl(txh, add, name);
 }
 
 void
@@ -957,7 +951,7 @@ dmu_tx_hold_bonus_by_dnode(dmu_tx_t *tx, dnode_t *dn)
 {
 	dmu_tx_hold_t *txh;
 
-	ASSERT(tx->tx_txg == 0);
+	ASSERT0(tx->tx_txg);
 
 	txh = dmu_tx_hold_dnode_impl(tx, dn, THT_BONUS, 0, 0);
 	if (txh)
diff --git a/module/zfs/zap_micro.c b/module/zfs/zap_micro.c
index 0c8ee9d25a..53f8d2313a 100644
--- a/module/zfs/zap_micro.c
+++ b/module/zfs/zap_micro.c
@@ -1207,7 +1207,8 @@ zap_add_impl(zap_t *zap, const char *key,
 	}
 	ASSERT(zap == zn->zn_zap);
 	zap_name_free(zn);
-	zap_unlockdir(zap, tag);
+	if (zap != NULL)	/* may be NULL if fzap_add() failed */
+		zap_unlockdir(zap, tag);
 	return (err);
 }