diff --git a/mysql-test/r/myisampack.result b/mysql-test/r/myisampack.result
index 5f39d318234..14b6283bf8f 100644
--- a/mysql-test/r/myisampack.result
+++ b/mysql-test/r/myisampack.result
@@ -27,3 +27,25 @@ CHECK TABLE t1 EXTENDED;
 Table	Op	Msg_type	Msg_text
 test.t1	check	status	OK
 DROP TABLE t1;
+drop table if exists t1;
+create table t1(f1 int, f2 varchar(255));
+insert into t1 values(1, 'foo'), (2, 'bar');
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+flush tables;
+optimize table t1;
+Table	Op	Msg_type	Msg_text
+test.t1	optimize	error	Table 'test.t1' is read only
+Warnings:
+Error	1036	Table 't1' is read only
+drop table t1;
diff --git a/mysql-test/t/myisampack.test b/mysql-test/t/myisampack.test
index 6598af6318a..99767f81033 100644
--- a/mysql-test/t/myisampack.test
+++ b/mysql-test/t/myisampack.test
@@ -31,3 +31,28 @@ FLUSH TABLES;
 --exec $MYISAMCHK -s --unpack $MYSQLTEST_VARDIR/master-data/test/t1
 CHECK TABLE t1 EXTENDED;
 DROP TABLE t1;
+
+#
+# Bug#40949 Debug version of MySQL server crashes when run OPTIMIZE on compressed table.
+#
+--disable_warnings
+drop table if exists t1;
+--enable_warnings
+create table t1(f1 int, f2 varchar(255));
+insert into t1 values(1, 'foo'), (2, 'bar');
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+insert into t1 select * from t1;
+flush tables;
+--exec $MYISAMPACK $MYSQLTEST_VARDIR/master-data/test/t1
+optimize table t1;
+drop table t1;
diff --git a/sql/sql_table.cc b/sql/sql_table.cc
index ec35616bc89..16d420441d1 100644
--- a/sql/sql_table.cc
+++ b/sql/sql_table.cc
@@ -4319,6 +4319,7 @@ static bool mysql_admin_table(THD* thd, TABLE_LIST* tables,
       table->table=0;				// For query cache
       if (protocol->write())
 	goto err;
+      thd->main_da.reset_diagnostics_area();
       continue;
       /* purecov: end */
     }