diff --git a/mysql-test/r/subselect_extra_no_semijoin.result b/mysql-test/r/subselect_extra_no_semijoin.result
index 79bca388181..200682b4065 100644
--- a/mysql-test/r/subselect_extra_no_semijoin.result
+++ b/mysql-test/r/subselect_extra_no_semijoin.result
@@ -482,3 +482,22 @@ DROP TABLE t1,t2;
 set optimizer_switch= @tmp_subselect_extra_derived;
 set optimizer_switch= @subselect_extra_no_sj_tmp;
 set @optimizer_switch_for_subselect_extra_test=null;
+#
+# MDEV-6439: Server crashes in Explain_union::print_explain with explain in slow log, tis620 charset
+#
+SET NAMES tis620;
+set @tmp= @@global.slow_query_log;
+SET GLOBAL slow_query_log = 1;
+SET long_query_time = 0.000001;
+SET log_slow_verbosity = 'explain';
+CREATE TABLE t1 (a VARCHAR(3)) ENGINE=MyISAM;
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo');
+a
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo' UNION SELECT 'bar' );
+ERROR HY000: Illegal mix of collations (tis620_thai_ci,COERCIBLE) and (latin1_swedish_ci,IMPLICIT) for operation '<='
+create table t2 (b int);
+insert into t2 values (1),(2),(3);
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo' FROM t2);
+ERROR HY000: Illegal mix of collations (tis620_thai_ci,COERCIBLE) and (latin1_swedish_ci,IMPLICIT) for operation '<='
+drop table t1,t2;
+SET GLOBAL slow_query_log=@tmp;
diff --git a/mysql-test/t/subselect_extra_no_semijoin.test b/mysql-test/t/subselect_extra_no_semijoin.test
index 8aba3dde72b..d8809c7f981 100644
--- a/mysql-test/t/subselect_extra_no_semijoin.test
+++ b/mysql-test/t/subselect_extra_no_semijoin.test
@@ -6,4 +6,33 @@ set  @optimizer_switch_for_subselect_extra_test='semijoin=off,firstmatch=off,loo
 
 set optimizer_switch= @subselect_extra_no_sj_tmp;
 
-set @optimizer_switch_for_subselect_extra_test=null;
\ No newline at end of file
+set @optimizer_switch_for_subselect_extra_test=null;
+
+--echo #
+--echo # MDEV-6439: Server crashes in Explain_union::print_explain with explain in slow log, tis620 charset
+--echo #
+
+## Using a separate client connection is easier than restoring state
+connect(con1,localhost,root,,);
+
+SET NAMES tis620;
+set @tmp= @@global.slow_query_log;
+SET GLOBAL slow_query_log = 1;
+SET long_query_time = 0.000001;
+SET log_slow_verbosity = 'explain';
+
+CREATE TABLE t1 (a VARCHAR(3)) ENGINE=MyISAM;
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo');
+--error ER_CANT_AGGREGATE_2COLLATIONS
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo' UNION SELECT 'bar' );
+
+create table t2 (b int);
+insert into t2 values (1),(2),(3);
+
+--error ER_CANT_AGGREGATE_2COLLATIONS
+SELECT * FROM t1 WHERE a >= ANY ( SELECT 'foo' FROM t2);
+
+drop table t1,t2;
+SET GLOBAL slow_query_log=@tmp;
+disconnect con1;
+connection default;
diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index a8dfdff9809..14b2ccd3985 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -575,7 +575,7 @@ bool Item_subselect::is_expensive()
     */
     if (cur_join->optimized &&
         (cur_join->zero_result_cause || !cur_join->tables_list))
-      return false;
+      continue;
 
     /*
       If a subquery is not optimized we cannot estimate its cost. A subquery is