Problem with Set TEXT field to NULL

Hello,

I use the following command to set a TEXT field to NULL in tsql or sqsh. It
is executed without error.

UPDATE table_a SET col_y = NULL WHERE col_id=1

Then if I use SELECT to see the result, it shows up like it is empty value,
not NULL. What's more, if I use php to SELECT this column, it will crash.

Part of the SELECT log by using php:

14:46:53.448043 3827 (token.c:510):processing result tokens.  marker is
d1(ROW)
14:46:53.448061 3827 (token.c:2166):tds_process_row(): reading column 0
14:46:53.448078 3827 (token.c:1915):tds_get_data: type 35, varint size 4
14:46:53.448097 3827 (token.c:1975):tds_get_data(): wire column size is 0
14:46:53.448114 3827 (util.c:162):Changed query state from READING to
PENDING
14:46:53.448132 3827 (ct.c:1555):inside ct_fetch()process_row_tokens
returned 1
14:46:53.448149 3827 (ct.c:1704):_ct_bind_data()
14:46:53.448166 3827 (ct.c:1711):_ct_bind_data(): column_type: 35
column_len: 0
14:46:53.448182 3827 (ct.c:2003):_ct_get_server_type(0)
14:46:53.448199 3827 (ct.c:1890):_ct_get_client_type(type 35, user 19, size
64512)
14:46:53.448220 3827 (cs.c:463):cs_convert()
14:46:53.448237 3827 (ct.c:2003):_ct_get_server_type(4)
14:46:53.448254 3827 (ct.c:2003):_ct_get_server_type(0)
14:46:53.448271 3827 (cs.c:470):cs_convert() srctype = 35 (0) desttype = 47
(64513)
14:46:53.448289 3827 (cs.c:483):cs_convert() srcdata is null

Please notice that wire column size is 0. If it is NULL, it should be -1.

As I have no problem to SELECT NULL or empty TEXT field, I suppose the
UPDATE is going something wrong.

Any ideas?

Thanks,

Min