update table by subquery

hi,

I need to update following table. but showing error... 

 update test_info_tag_ext  a

set file_name = (select real_filename from ec_blob b where a.test_instance_id=b.test_instance_id)

where  a.test_instance_id in (select

     eb.test_instance_id

   -- tie.file_name,

   -- tie.report_filename,

   -- eb.real_filename,

   -- eb.seq_number,

   -- bt.type_name

from

    ec_blob eb,

    blob_types bt,

    v_optics_test_file_extensions otfe,

    test_info_tag_ext tie

where

    eb.type = bt.type_id

and lower(bt.type_name) = lower(otfe.file_extension)

and eb.seq_number = 1