diff --git a/mysql-test/r/ps.result b/mysql-test/r/ps.result
index 1cd41e5b3a4..81167bc85c0 100644
--- a/mysql-test/r/ps.result
+++ b/mysql-test/r/ps.result
@@ -5487,5 +5487,27 @@ a
 DEALLOCATE PREPARE stmt;
 DROP VIEW v1;
 DROP TABLE t1;
+#
+# MDEV-19631: Assertion `0' failed in st_select_lex_unit::optimize or
+#             different plan upon 2nd execution of PS with EXPLAIN
+#
+CREATE TABLE t1 (a INT);
+PREPARE stmt FROM 'EXPLAIN SELECT * FROM t1 HAVING 6 IN ( SELECT 6 UNION SELECT 5 )';
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	0	const row not found
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+# Without the patch the second execution of the 'stmt' prepared statement
+# would result in server crash.
+EXECUTE stmt;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t1	system	NULL	NULL	NULL	NULL	0	const row not found
+2	SUBQUERY	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+3	UNION	NULL	NULL	NULL	NULL	NULL	NULL	NULL	No tables used
+NULL	UNION RESULT	<union2,3>	ALL	NULL	NULL	NULL	NULL	NULL	
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
 # End of 10.2 tests
 #
diff --git a/mysql-test/t/ps.test b/mysql-test/t/ps.test
index 4097d28a949..81d8e8e0fed 100644
--- a/mysql-test/t/ps.test
+++ b/mysql-test/t/ps.test
@@ -4976,6 +4976,19 @@ DEALLOCATE PREPARE stmt;
 DROP VIEW v1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-19631: Assertion `0' failed in st_select_lex_unit::optimize or
+--echo #             different plan upon 2nd execution of PS with EXPLAIN
+--echo #
+CREATE TABLE t1 (a INT);
+PREPARE stmt FROM 'EXPLAIN SELECT * FROM t1 HAVING 6 IN ( SELECT 6 UNION SELECT 5 )';
+EXECUTE stmt;
+--echo # Without the patch the second execution of the 'stmt' prepared statement
+--echo # would result in server crash.
+EXECUTE stmt;
+# Cleanup
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
 
 --echo # End of 10.2 tests
 --echo #
diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index a3e8b453897..22d3597de16 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -25400,14 +25400,20 @@ bool mysql_explain_union(THD *thd, SELECT_LEX_UNIT *unit, select_result *result)
 
   if (unit->is_union() || unit->fake_select_lex)
   {
+    ulonglong save_options= 0;
+
     if (unit->union_needs_tmp_table() && unit->fake_select_lex)
     {
+      save_options= unit->fake_select_lex->options;
       unit->fake_select_lex->select_number= FAKE_SELECT_LEX_ID; // just for initialization
       unit->fake_select_lex->type= "UNION RESULT";
       unit->fake_select_lex->options|= SELECT_DESCRIBE;
     }
     if (!(res= unit->prepare(thd, result, SELECT_NO_UNLOCK | SELECT_DESCRIBE)))
       res= unit->exec();
+
+    if (unit->union_needs_tmp_table() && unit->fake_select_lex)
+      unit->fake_select_lex->options= save_options;
   }
   else
   {