diff --git a/mysql-test/main/mdev_39918.result b/mysql-test/main/mdev_39918.result
new file mode 100644
index 0000000000000..f330ea9f837c4
--- /dev/null
+++ b/mysql-test/main/mdev_39918.result
@@ -0,0 +1,60 @@
+#
+# MDEV-39918: Crash in execute_degenerate_jtbm_semi_join with non-SINGLE_SELECT_ENGINE
+#
+# Recursive CTE with window function (original crash)
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (
+WITH RECURSIVE x(x) AS (SELECT 10 AS x)
+SELECT ROW_NUMBER() OVER (ORDER BY AVG(x))
+FROM (SELECT 1 AS x WHERE 1=0) AS d WHERE x
+);
+1
+1
+# Truly recursive CTE nested in derived table
+SELECT 1 FROM (SELECT 1 AS x) AS outer_t
+WHERE x IN (
+SELECT ROW_NUMBER() OVER (ORDER BY AVG(x))
+FROM (
+WITH RECURSIVE x(x) AS (SELECT 1 UNION SELECT x + 1 FROM x WHERE x < 3)
+SELECT * FROM x WHERE 1=0
+) AS d WHERE x
+);
+1
+1
+# Simple UNION subquery (no tables in first SELECT)
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 UNION SELECT 2);
+1
+1
+# UNION ALL with degenerate first branch
+SELECT * FROM (SELECT 1 AS x UNION ALL SELECT 2) AS t
+WHERE x IN (SELECT 1 UNION ALL SELECT 2);
+x
+1
+2
+# EXCEPT variant
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 EXCEPT SELECT 2);
+1
+1
+# INTERSECT variant
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 INTERSECT SELECT 1);
+1
+1
+# Non-degenerate subquery (has tables, should work as before)
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2), (3);
+SELECT * FROM t1 WHERE a IN (SELECT a FROM t1 WHERE a > 1);
+a
+2
+3
+DROP TABLE t1;
+# UNION with table in one branch, no table in another
+CREATE TABLE t2 (a INT);
+INSERT INTO t2 VALUES (5);
+SELECT * FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 UNION SELECT a FROM t2);
+x
+1
+DROP TABLE t2;
diff --git a/mysql-test/main/mdev_39918.test b/mysql-test/main/mdev_39918.test
new file mode 100644
index 0000000000000..cc9d8f4d37fe1
--- /dev/null
+++ b/mysql-test/main/mdev_39918.test
@@ -0,0 +1,56 @@
+#
+# MDEV-39918: MariaDB crash triggered by recursive CTE with window function
+# in IN subquery (degenerate jtbm semi-join)
+#
+
+--echo #
+--echo # MDEV-39918: Crash in execute_degenerate_jtbm_semi_join with non-SINGLE_SELECT_ENGINE
+--echo #
+
+--echo # Recursive CTE with window function (original crash)
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (
+  WITH RECURSIVE x(x) AS (SELECT 10 AS x)
+  SELECT ROW_NUMBER() OVER (ORDER BY AVG(x))
+  FROM (SELECT 1 AS x WHERE 1=0) AS d WHERE x
+);
+
+--echo # Truly recursive CTE nested in derived table
+SELECT 1 FROM (SELECT 1 AS x) AS outer_t
+WHERE x IN (
+  SELECT ROW_NUMBER() OVER (ORDER BY AVG(x))
+  FROM (
+    WITH RECURSIVE x(x) AS (SELECT 1 UNION SELECT x + 1 FROM x WHERE x < 3)
+    SELECT * FROM x WHERE 1=0
+  ) AS d WHERE x
+);
+
+--echo # Simple UNION subquery (no tables in first SELECT)
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 UNION SELECT 2);
+
+--echo # UNION ALL with degenerate first branch
+SELECT * FROM (SELECT 1 AS x UNION ALL SELECT 2) AS t
+WHERE x IN (SELECT 1 UNION ALL SELECT 2);
+
+--echo # EXCEPT variant
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 EXCEPT SELECT 2);
+
+--echo # INTERSECT variant
+SELECT 1 FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 INTERSECT SELECT 1);
+
+--echo # Non-degenerate subquery (has tables, should work as before)
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1), (2), (3);
+SELECT * FROM t1 WHERE a IN (SELECT a FROM t1 WHERE a > 1);
+DROP TABLE t1;
+
+--echo # UNION with table in one branch, no table in another
+CREATE TABLE t2 (a INT);
+INSERT INTO t2 VALUES (5);
+SELECT * FROM (SELECT 1 AS x) AS t
+WHERE x IN (SELECT 1 UNION SELECT a FROM t2);
+DROP TABLE t2;
+
diff --git a/sql/opt_subselect.cc b/sql/opt_subselect.cc
index bbc9434c4cf02..78e0e05072746 100644
--- a/sql/opt_subselect.cc
+++ b/sql/opt_subselect.cc
@@ -6552,7 +6552,15 @@ bool setup_degenerate_jtbm_semi_joins(JOIN *join,
     {
       JOIN *subq_join= subq_pred->unit->first_select()->join;
 
-      if (!subq_join->tables_list || !subq_join->table_count)
+      /*
+        The degenerate path requires SINGLE_SELECT_ENGINE.
+        Other engine types (UNION_ENGINE for UNION/EXCEPT/INTERSECT/
+        recursive CTEs, etc.) must go through normal JTBM
+        materialization instead.
+      */
+      if ((!subq_join->tables_list || !subq_join->table_count) &&
+          subq_pred->engine->engine_type() ==
+          subselect_engine::SINGLE_SELECT_ENGINE)
       {
         if (execute_degenerate_jtbm_semi_join(thd,
                                               table,
@@ -6645,7 +6653,15 @@ bool setup_jtbm_semi_joins(JOIN *join, List<TABLE_LIST> *join_list,
       subq_pred->jtbm_record_count=rows;
       JOIN *subq_join= subq_pred->unit->first_select()->join;
 
-      if (!subq_join->tables_list || !subq_join->table_count)
+      /*
+        The degenerate path requires SINGLE_SELECT_ENGINE.
+        Other engine types (UNION_ENGINE for UNION/EXCEPT/INTERSECT/
+        recursive CTEs, etc.) must go through normal JTBM
+        materialization instead.
+      */
+      if ((!subq_join->tables_list || !subq_join->table_count) &&
+          subq_pred->engine->engine_type() ==
+          subselect_engine::SINGLE_SELECT_ENGINE)
       {
         if (!join->is_orig_degenerated &&
             execute_degenerate_jtbm_semi_join(thd, table, subq_pred,