diff --git a/mysql-test/r/select.result b/mysql-test/r/select.result
index c2218585f7c..69228d2ff44 100644
--- a/mysql-test/r/select.result
+++ b/mysql-test/r/select.result
@@ -3125,6 +3125,32 @@ select count(*)
 from t1 inner join (t2 right join t3 on t2.id = t3.b_id) on t1.id = t3.a_id;
 count(*)
 6
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,1), (2,1), (4,10);
+CREATE TABLE t2 (a int PRIMARY KEY, b int, KEY b (b));
+INSERT INTO t2 VALUES (1,NULL), (2,10);
+ALTER TABLE t1 ENABLE KEYS;
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	index	b	b	5	NULL	2	Using index
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+a	b	a	b
+1	NULL	1	1
+1	NULL	2	1
+1	NULL	4	10
+2	10	4	10
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t2	index	b	b	5	NULL	2	Using index
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	3	Using where
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+a	b	a	b
+1	NULL	1	1
+1	NULL	2	1
+1	NULL	4	10
+2	10	4	10
+DROP TABLE IF EXISTS t1,t2;
 drop table t1,t2,t3;
 create table t1 (a int);
 create table t2 (b int);
diff --git a/mysql-test/t/select.test b/mysql-test/t/select.test
index 592e366f835..a55645bcc7c 100644
--- a/mysql-test/t/select.test
+++ b/mysql-test/t/select.test
@@ -2304,6 +2304,21 @@ INSERT INTO t1 VALUES (10);
 SELECT i='1e+01',i=1e+01, i in (1e+01,1e+01), i in ('1e+01','1e+01') FROM t1;
 DROP TABLE t1;
 
+#
+# Bug #21019: First result of SELECT COUNT(*) different than consecutive runs
+#
+CREATE TABLE t1 (a int, b int);
+INSERT INTO t1 VALUES (1,1), (2,1), (4,10);
+
+CREATE TABLE t2 (a int PRIMARY KEY, b int, KEY b (b));
+INSERT INTO t2 VALUES (1,NULL), (2,10);
+ALTER TABLE t1 ENABLE KEYS;
+
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+EXPLAIN SELECT STRAIGHT_JOIN SQL_NO_CACHE COUNT(*) FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+SELECT STRAIGHT_JOIN SQL_NO_CACHE * FROM t2, t1 WHERE t1.b = t2.b OR t2.b IS NULL;
+DROP TABLE IF EXISTS t1,t2;
 # End of 4.1 tests
 
 #
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index 36962a233b8..5c812e759e6 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -2490,8 +2490,11 @@ merge_key_fields(KEY_FIELD *start,KEY_FIELD *new_fields,KEY_FIELD *end,
 	  /* field = expression OR field IS NULL */
 	  old->level= and_level;
 	  old->optimize= KEY_OPTIMIZE_REF_OR_NULL;
-	  /* Remember the NOT NULL value */
-	  if (old->val->is_null())
+	  /*
+            Remember the NOT NULL value unless the value does not depend
+            on other tables.
+          */
+	  if (!old->val->used_tables() && old->val->is_null())
 	    old->val= new_fields->val;
           /* The referred expression can be NULL: */ 
           old->null_rejecting= 0;