diff --git a/mysql-test/suite/innodb_fts/r/create.result b/mysql-test/suite/innodb_fts/r/create.result
index 615f0368599..4e21c080bbb 100644
--- a/mysql-test/suite/innodb_fts/r/create.result
+++ b/mysql-test/suite/innodb_fts/r/create.result
@@ -166,3 +166,19 @@ SELECT len,COUNT(*) FROM INFORMATION_SCHEMA.INNODB_SYS_COLUMNS where name='word'
 len	COUNT(*)
 84	6
 DROP TABLE t;
+#
+# MDEV-17923 Assertion memcmp(field, field_ref_zero, 7) failed in
+# trx_undo_page_report_modify upon optimizing table
+# under innodb_optimize_fulltext_only
+#
+CREATE TABLE t1 (f1 TEXT, f2 TEXT, FULLTEXT KEY (f2)) ENGINE=InnoDB;
+INSERT INTO t1 (f1) VALUES ('foo'),('bar');
+DELETE FROM t1 LIMIT 1;
+ALTER TABLE t1 ADD FULLTEXT KEY (f1);
+SET @optimize_fulltext.save= @@innodb_optimize_fulltext_only;
+SET GLOBAL innodb_optimize_fulltext_only= 1;
+OPTIMIZE TABLE t1;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	status	OK
+DROP TABLE t1;
+SET GLOBAL innodb_optimize_fulltext_only= @optimize_fulltext.save;
diff --git a/mysql-test/suite/innodb_fts/t/create.test b/mysql-test/suite/innodb_fts/t/create.test
index fe0273af750..4e522994fcc 100644
--- a/mysql-test/suite/innodb_fts/t/create.test
+++ b/mysql-test/suite/innodb_fts/t/create.test
@@ -90,3 +90,19 @@ ENGINE=InnoDB;
 # The column length should be 84 bytes (84 characters * 1 byte/character).
 SELECT len,COUNT(*) FROM INFORMATION_SCHEMA.INNODB_SYS_COLUMNS where name='word' GROUP BY len;
 DROP TABLE t;
+
+--echo #
+--echo # MDEV-17923 Assertion memcmp(field, field_ref_zero, 7) failed in
+--echo # trx_undo_page_report_modify upon optimizing table
+--echo # under innodb_optimize_fulltext_only
+--echo #
+
+CREATE TABLE t1 (f1 TEXT, f2 TEXT, FULLTEXT KEY (f2)) ENGINE=InnoDB;
+INSERT INTO t1 (f1) VALUES ('foo'),('bar');
+DELETE FROM t1 LIMIT 1;
+ALTER TABLE t1 ADD FULLTEXT KEY (f1);
+SET @optimize_fulltext.save= @@innodb_optimize_fulltext_only;
+SET GLOBAL innodb_optimize_fulltext_only= 1;
+OPTIMIZE TABLE t1;
+DROP TABLE t1;
+SET GLOBAL innodb_optimize_fulltext_only= @optimize_fulltext.save;
diff --git a/storage/innobase/row/row0ftsort.cc b/storage/innobase/row/row0ftsort.cc
index 810b5387aec..d5eee7e0727 100644
--- a/storage/innobase/row/row0ftsort.cc
+++ b/storage/innobase/row/row0ftsort.cc
@@ -1581,9 +1581,7 @@ row_fts_merge_insert(
 	dict_table_t*		aux_table;
 	dict_index_t*		aux_index;
 	trx_t*			trx;
-	byte			trx_id_buf[6];
-	roll_ptr_t		roll_ptr = 0;
-	dfield_t*		field;
+	byte			sys_buf[DATA_TRX_ID_LEN + DATA_ROLL_PTR_LEN];
 
 	ut_ad(index);
 	ut_ad(table);
@@ -1689,16 +1687,16 @@ row_fts_merge_insert(
 			      dict_index_get_n_fields(aux_index));
 
 	/* Set TRX_ID and ROLL_PTR */
-	trx_write_trx_id(trx_id_buf, trx->id);
-	field = dtuple_get_nth_field(ins_ctx.tuple, 2);
-	dfield_set_data(field, &trx_id_buf, 6);
+	trx_write_trx_id(sys_buf, trx->id);
+	trx_write_roll_ptr(sys_buf + DATA_TRX_ID_LEN,
+			   1ULL << ROLL_PTR_INSERT_FLAG_POS);
+	dfield_set_data(dtuple_get_nth_field(ins_ctx.tuple, 2),
+			&sys_buf, DATA_TRX_ID_LEN);
+	dfield_set_data(dtuple_get_nth_field(ins_ctx.tuple, 3),
+			&sys_buf + DATA_TRX_ID_LEN, DATA_ROLL_PTR_LEN);
 
-	field = dtuple_get_nth_field(ins_ctx.tuple, 3);
-	dfield_set_data(field, &roll_ptr, 7);
+	ut_d(ins_ctx.aux_index_id = id);
 
-#ifdef UNIV_DEBUG
-	ins_ctx.aux_index_id = id;
-#endif
 	const ulint space = table->space;
 
 	for (i = 0; i < fts_sort_pll_degree; i++) {