diff --git a/mysql-test/main/log_slow.result b/mysql-test/main/log_slow.result
index 31d52ff0323..6b9fddb7fb6 100644
--- a/mysql-test/main/log_slow.result
+++ b/mysql-test/main/log_slow.result
@@ -64,7 +64,7 @@ rows_affected	int(11)	NO		NULL
 flush slow logs;
 set long_query_time=0.1;
 set log_slow_filter='';
-set global slow_query_log=1;
+set slow_query_log=1;
 set global log_output='TABLE';
 select sleep(0.5);
 sleep(0.5)
@@ -73,7 +73,7 @@ select count(*) FROM mysql.slow_log;
 count(*)
 1
 set @@long_query_time=default;
-set global slow_query_log= @org_slow_query_log;
+set @@slow_query_log=default;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;
 set global log_output= default;
@@ -115,3 +115,21 @@ Slow_queries_increment
 SET log_slow_filter=DEFAULT;
 SET @@long_query_time=default;
 SET GLOBAL slow_query_log= @org_slow_query_log;
+#
+# MDEV-21187: log_slow_filter="" logs queries not using indexes
+#
+flush status;
+create table t (id int);
+insert into t values (1),(4);
+set log_slow_filter='';
+select * from t;
+id
+1
+4
+show session status like 'Slow_queries';
+Variable_name	Value
+Slow_queries	0
+drop table t;
+#
+# End of 10.3 tests
+#
diff --git a/mysql-test/main/log_slow.test b/mysql-test/main/log_slow.test
index b51777d859e..d2e314cf667 100644
--- a/mysql-test/main/log_slow.test
+++ b/mysql-test/main/log_slow.test
@@ -50,14 +50,14 @@ flush slow logs;
 # MDEV-4206 (empty filter should be no filter)
 set long_query_time=0.1;
 set log_slow_filter='';
-set global slow_query_log=1;
+set slow_query_log=1;
 set global log_output='TABLE';
 select sleep(0.5);
 select count(*) FROM mysql.slow_log;
 
 # Reset used variables
 set @@long_query_time=default;
-set global slow_query_log= @org_slow_query_log;
+set @@slow_query_log=default;
 set @@log_slow_filter=default;
 set @@log_slow_verbosity=default;
 set global log_output= default;
@@ -102,3 +102,20 @@ SET log_slow_filter=DEFAULT;
 
 SET @@long_query_time=default;
 SET GLOBAL slow_query_log= @org_slow_query_log;
+
+--echo #
+--echo # MDEV-21187: log_slow_filter="" logs queries not using indexes
+--echo #
+
+flush status;
+create table t (id int);
+insert into t values (1),(4);
+set log_slow_filter='';
+select * from t;
+show session status like 'Slow_queries';
+
+drop table t;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #
diff --git a/sql/sql_parse.cc b/sql/sql_parse.cc
index 385360168a1..cf316c8cc1c 100644
--- a/sql/sql_parse.cc
+++ b/sql/sql_parse.cc
@@ -2513,7 +2513,7 @@ void log_slow_statement(THD *thd)
   if ((thd->server_status &
        (SERVER_QUERY_NO_INDEX_USED | SERVER_QUERY_NO_GOOD_INDEX_USED)) &&
       !(thd->query_plan_flags & QPLAN_STATUS) &&
-      !slow_filter_masked(thd, QPLAN_NOT_USING_INDEX))
+      (thd->variables.log_slow_filter & QPLAN_NOT_USING_INDEX))
   {
     thd->query_plan_flags|= QPLAN_NOT_USING_INDEX;
     /* We are always logging no index queries if enabled in filter */