diff --git a/mysql-test/r/subselect.result b/mysql-test/r/subselect.result
index 57d6199675d..55d48030a07 100644
--- a/mysql-test/r/subselect.result
+++ b/mysql-test/r/subselect.result
@@ -3002,6 +3002,21 @@ field1	field2
 1	1
 1	3
 DROP TABLE t1, t2;
+CREATE TABLE t1(a int, INDEX (a));
+INSERT INTO t1 VALUES (1), (3), (5), (7);
+INSERT INTO t1 VALUES (NULL);
+CREATE TABLE t2(a int);
+INSERT INTO t2 VALUES (1),(2),(3);
+EXPLAIN SELECT a, a IN (SELECT a FROM t1) FROM t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	3	
+2	DEPENDENT SUBQUERY	t1	index_subquery	a	a	5	func	2	Using index
+SELECT a, a IN (SELECT a FROM t1) FROM t2;
+a	a IN (SELECT a FROM t1)
+1	1
+2	NULL
+3	1
+DROP TABLE t1,t2;
 create table t1 (df decimal(5,1));
 insert into t1 values(1.1);
 insert into t1 values(2.2);
diff --git a/mysql-test/t/subselect.test b/mysql-test/t/subselect.test
index 6d5082c360b..29316e6e6a0 100644
--- a/mysql-test/t/subselect.test
+++ b/mysql-test/t/subselect.test
@@ -1955,6 +1955,23 @@ SELECT field1, field2
 
 DROP TABLE t1, t2;
 
+#
+# Bug #23478: not top-level IN subquery returning a non-empty result set
+#             with possible NULL values by index access from the outer query
+#
+
+CREATE TABLE t1(a int, INDEX (a));
+INSERT INTO t1 VALUES (1), (3), (5), (7);
+INSERT INTO t1 VALUES (NULL);
+
+CREATE TABLE t2(a int);
+INSERT INTO t2 VALUES (1),(2),(3);
+
+EXPLAIN SELECT a, a IN (SELECT a FROM t1) FROM t2;
+SELECT a, a IN (SELECT a FROM t1) FROM t2;
+
+DROP TABLE t1,t2;
+
 # End of 4.1 tests
 
 #
diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index 0ad517609c9..78dae0aaa1e 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -667,6 +667,7 @@ double Item_in_subselect::val_real()
   */
   DBUG_ASSERT(0);
   DBUG_ASSERT(fixed == 1);
+  null_value= 0;
   if (exec())
   {
     reset();
@@ -687,6 +688,7 @@ longlong Item_in_subselect::val_int()
   */
   DBUG_ASSERT(0);
   DBUG_ASSERT(fixed == 1);
+  null_value= 0;
   if (exec())
   {
     reset();
@@ -707,6 +709,7 @@ String *Item_in_subselect::val_str(String *str)
   */
   DBUG_ASSERT(0);
   DBUG_ASSERT(fixed == 1);
+  null_value= 0;
   if (exec())
   {
     reset();