diff --git a/mysql-test/r/partition_alter.result b/mysql-test/r/partition_alter.result
index 6aa0039c8d2..e42604ba056 100644
--- a/mysql-test/r/partition_alter.result
+++ b/mysql-test/r/partition_alter.result
@@ -120,3 +120,10 @@ t1	CREATE TABLE `t1` (
  PARTITION `p02` ENGINE = MyISAM,
  PARTITION `p03` ENGINE = MyISAM)
 drop table t1;
+create or replace table t1 (x int) partition by hash (x) (partition p1, partition p2);
+lock table t1 write;
+alter table t1 add partition (partition p1);
+ERROR HY000: Duplicate partition name p1
+alter table t1 add partition (partition p1);
+ERROR HY000: Duplicate partition name p1
+drop table t1;
diff --git a/mysql-test/t/partition_alter.test b/mysql-test/t/partition_alter.test
index ce6672c4b9d..62a3e1c0777 100644
--- a/mysql-test/t/partition_alter.test
+++ b/mysql-test/t/partition_alter.test
@@ -114,3 +114,14 @@ insert into t1 values(0, 1, 1, NULL, now(), now());
 alter online table t1 delay_key_write=1;
 show create table t1;
 drop table t1;
+
+#
+# MDEV-14817 Server crashes in prep_alter_part_table() after table lock and multiple add partition
+#
+create or replace table t1 (x int) partition by hash (x) (partition p1, partition p2);
+lock table t1 write;
+--error ER_SAME_NAME_PARTITION
+alter table t1 add partition (partition p1);
+--error ER_SAME_NAME_PARTITION
+alter table t1 add partition (partition p1);
+drop table t1;
diff --git a/sql/sql_class.h b/sql/sql_class.h
index b56729737de..fddd736fc56 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -1797,8 +1797,9 @@ private:
 
 class Locked_tables_list
 {
-private:
+public:
   MEM_ROOT m_locked_tables_root;
+private:
   TABLE_LIST *m_locked_tables;
   TABLE_LIST **m_locked_tables_last;
   /** An auxiliary array used only in reopen_tables(). */
diff --git a/sql/sql_partition.cc b/sql/sql_partition.cc
index 879a29f4e42..f5fd7c891d8 100644
--- a/sql/sql_partition.cc
+++ b/sql/sql_partition.cc
@@ -4573,6 +4573,8 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
   /* ALTER_ADMIN_PARTITION is handled in mysql_admin_table */
   DBUG_ASSERT(!(alter_info->flags & Alter_info::ALTER_ADMIN_PARTITION));
 
+  partition_info *saved_part_info= NULL;
+
   if (alter_info->flags &
       (Alter_info::ALTER_ADD_PARTITION |
        Alter_info::ALTER_DROP_PARTITION |
@@ -4778,6 +4780,16 @@ uint prep_alter_part_table(THD *thd, TABLE *table, Alter_info *alter_info,
     }
     if (alter_info->flags & Alter_info::ALTER_ADD_PARTITION)
     {
+      if (*fast_alter_table && thd->locked_tables_mode)
+      {
+        MEM_ROOT *old_root= thd->mem_root;
+        thd->mem_root= &thd->locked_tables_list.m_locked_tables_root;
+        saved_part_info= tab_part_info->get_clone(thd);
+        thd->mem_root= old_root;
+        saved_part_info->read_partitions= tab_part_info->read_partitions;
+        saved_part_info->lock_partitions= tab_part_info->lock_partitions;
+        saved_part_info->bitmaps_are_initialized= tab_part_info->bitmaps_are_initialized;
+      }
       /*
         We start by moving the new partitions to the list of temporary
         partitions. We will then check that the new partitions fit in the
@@ -5472,7 +5484,7 @@ the generated partition syntax in a correct manner.
         goto err;
       }
     }
-  }
+  } // ADD, DROP, COALESCE, REORGANIZE, TABLE_REORG, REBUILD
   else
   {
     /*
@@ -5638,6 +5650,8 @@ the generated partition syntax in a correct manner.
   DBUG_RETURN(FALSE);
 err:
   *fast_alter_table= false;
+  if (saved_part_info)
+    table->part_info= saved_part_info;
   DBUG_RETURN(TRUE);
 }