diff --git a/tests/unit/mock_dmu.c b/tests/unit/mock_dmu.c
index 65c38c1fd9f..ae035498da6 100644
--- a/tests/unit/mock_dmu.c
+++ b/tests/unit/mock_dmu.c
@@ -28,6 +28,7 @@
 #include <sys/zfeature.h>
 
 #include "mock_dmu.h"
+#include "unit.h"
 
 /*
  * A mock dbuf. A real dmu_buf_t (first for casting) plus the attached user
@@ -48,6 +49,7 @@ typedef struct mock_dbuf mock_dbuf_t;
  */
 struct mock_dnode {
 	dnode_t			mdn_dn;
+	uint64_t		mdn_refcount;
 	size_t			mdn_blksize;
 	size_t			mdn_nblocks;
 	mock_dbuf_t		**mdn_blocks;
@@ -110,6 +112,7 @@ mock_dnode_create(size_t blksize, dmu_object_type_t type)
 	ASSERT(IS_P2ALIGNED(blksize, 512));
 
 	mock_dnode_t *mdn = kmem_zalloc(sizeof (mock_dnode_t), KM_SLEEP);
+	mdn->mdn_refcount = 1;
 	mdn->mdn_dn.dn_type = type;
 	mdn->mdn_dn.dn_object = 1;	/* arbitrary non-zero object number */
 	mdn->mdn_blksize = blksize;
@@ -156,6 +159,12 @@ mock_dnode_block_data(mock_dnode_t *mdn, uint64_t blkid)
 	return (mdn->mdn_blocks[blkid]->mdb_db.db_data);
 }
 
+uint64_t
+mock_dnode_refcount(mock_dnode_t *mdn)
+{
+	return (mdn->mdn_refcount);
+}
+
 /* Mock transaction */
 
 mock_dmu_tx_t *
@@ -258,14 +267,21 @@ dmu_object_set_blocksize(objset_t *os, uint64_t object, uint64_t size,
 boolean_t
 dnode_add_ref(dnode_t *dn, const void *tag)
 {
-	(void) dn; (void) tag;
+	(void) tag;
+	mock_dnode_t *mdn = (mock_dnode_t *)dn;
+	if (mdn->mdn_refcount == 0)
+		return (B_FALSE);
+	mdn->mdn_refcount++;
 	return (B_TRUE);
 }
 
 void
 dnode_rele(dnode_t *dn, const void *tag)
 {
-	(void) dn; (void) tag;
+	(void) tag;
+	mock_dnode_t *mdn = (mock_dnode_t *)dn;
+	unit_gt(mdn->mdn_refcount, 0);
+	mdn->mdn_refcount--;
 }
 
 /*
diff --git a/tests/unit/mock_dmu.h b/tests/unit/mock_dmu.h
index a46454c779f..2ac82c18b7a 100644
--- a/tests/unit/mock_dmu.h
+++ b/tests/unit/mock_dmu.h
@@ -40,6 +40,9 @@ size_t mock_dnode_block_count(mock_dnode_t *mdn);
 /* Returns a pointer to the data under the given block id. */
 const void *mock_dnode_block_data(mock_dnode_t *mdn, uint64_t blkid);
 
+/* Returns the current dnode ref (hold) count. */
+uint64_t mock_dnode_refcount(mock_dnode_t *mdn);
+
 /* Create/destroy a mock transaction handle. */
 mock_dmu_tx_t *mock_tx_create(void);
 void mock_tx_destroy(mock_dmu_tx_t *tx);