Problem with casting BLOB to varchar

[ry1633]

In one of my FM query subjects I'm trying to cast a BLOB to varchar.   This is my expression:

CAST([Database Layer].[my table].[field_data_bytes], varchar(255))

Cognos seemingly will accept that as is - the blue arrow will light up and show.  But when I try to run and validate it, I get the following error.  What am I doing wrong and is there a better way to convert a BLOB?

RQP-DEF-0177 An error occurred while performing operation 'sqlPrepareWithOptions' status='-9'.
UDA-SQL-0107 A general exception has occurred during the operation "prepare".
ORA-00904: "coguda#cast_varchar": invalid identifier

[the6campbells]

open a PMR with IBM support as the query engine is generating an invalid statement which ORACLE is rejecting.

ORACLE itself provides packages with built-in functions which can allow you to type convert a BLOB to varchar format.

see https://docs.oracle.com/database/121/ARPLS/d_lob.htm

[Invisi]

Why does your data mart contain a blob column that you want to convert to a regular varchar in your visualisation tool metalayer?