diff --git a/mysql-test/suite/innodb_fts/r/create.result b/mysql-test/suite/innodb_fts/r/create.result
index 04fc20195b6b3..1147b26cfd2e3 100644
--- a/mysql-test/suite/innodb_fts/r/create.result
+++ b/mysql-test/suite/innodb_fts/r/create.result
@@ -211,3 +211,11 @@ ERROR 0A000: ALGORITHM=INPLACE is not supported. Reason: Not implemented for sys
 ALTER TABLE t1 ADD FULLTEXT INDEX(b), ALGORITHM=COPY;
 ERROR HY000: Index 'FTS_DOC_ID_INDEX' is of wrong type for an InnoDB FULLTEXT index
 DROP TABLE t1;
+#
+# MDEV-40985 row_end access out of bounds in ALTER TABLE
+#
+CREATE TABLE t1
+(a TEXT,b INT AS(1),c INT AS(1),d INT AS(1),e INT AS(1),f INT AS(1),
+FULLTEXT INDEX(a)) ENGINE=InnoDB;
+ALTER TABLE t1 FORCE;
+DROP TABLE t1;
diff --git a/mysql-test/suite/innodb_fts/t/create.test b/mysql-test/suite/innodb_fts/t/create.test
index 9d88c1eddd442..e4f215150774a 100644
--- a/mysql-test/suite/innodb_fts/t/create.test
+++ b/mysql-test/suite/innodb_fts/t/create.test
@@ -142,3 +142,13 @@ ALTER TABLE t1 ADD FULLTEXT INDEX(b), ALGORITHM=INPLACE;
 --error ER_INNODB_FT_WRONG_DOCID_INDEX
 ALTER TABLE t1 ADD FULLTEXT INDEX(b), ALGORITHM=COPY;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-40985 row_end access out of bounds in ALTER TABLE
+--echo #
+
+CREATE TABLE t1
+(a TEXT,b INT AS(1),c INT AS(1),d INT AS(1),e INT AS(1),f INT AS(1),
+ FULLTEXT INDEX(a)) ENGINE=InnoDB;
+ALTER TABLE t1 FORCE;
+DROP TABLE t1;
diff --git a/storage/innobase/handler/handler0alter.cc b/storage/innobase/handler/handler0alter.cc
index 99413f79c93b4..4a48168041aa9 100644
--- a/storage/innobase/handler/handler0alter.cc
+++ b/storage/innobase/handler/handler0alter.cc
@@ -4310,20 +4310,19 @@ ha_innobase_inplace_ctx::create_key_defs(
 
 	if (add_fts_doc_idx) {
 		index_def_t*	index = indexdef++;
-		uint nfields = 1;
-
-		if (altered_table->versioned())
-			++nfields;
+		const uint	nfields = altered_table->versioned() + 1;
 		index->fields = static_cast<index_field_t*>(
-			mem_heap_alloc(heap, sizeof(*index->fields) * nfields));
+			mem_heap_alloc(heap, nfields * sizeof *index->fields));
 		index->n_fields = nfields;
 		index->fields[0].col_no = fts_doc_id_col;
 		index->fields[0].prefix_len = 0;
 		index->fields[0].descending = false;
 		index->fields[0].is_v_col = false;
 		if (nfields == 2) {
-			index->fields[1].col_no
-				= altered_table->s->vers.end_fieldno;
+			/* FTS_DOC_ID_INDEX(FTS_DOC_ID,row_end) */
+			index->fields[1].col_no = innodb_col_no(
+				altered_table->field[
+					altered_table->s->vers.end_fieldno]);
 			index->fields[1].prefix_len = 0;
 			index->fields[1].descending = false;
 			index->fields[1].is_v_col = false;