Raise signal with dynamic message_text

Environment is DB2/LINUX 9.5.0.

I wonder if it is possible to set the MESSAGE_TEXT dynamically when
using the SIGNAL statement.  The objective is to return more specific
information to the calling user or application. E.g. I would rather
return "Unknown value 42" than just "Unknown value".

The documentation says:

http://publib.boulder.ibm.com/infocenter/db2luw/v9/index.jsp?topic=/com.ibm.db2.udb.admin.doc/doc/r0004232.htm

  SET MESSAGE_TEXT =

  diagnostic-string-expression

  An expression of type CHAR or VARCHAR that returns a character
  string of up to 70 bytes to describe the error condition. If the
  string is longer than 70 bytes, it is truncated.

So i thought something like this should work, but it does not (for me,
at least):

CREATE PROCEDURE customerror ()
BEGIN
  DECLARE v_value CHAR(5) DEFAULT 'wrong';
  DECLARE c_unknown CONDITION FOR SQLSTATE '90001';
  SIGNAL c_unknown SET MESSAGE_TEXT = ('Unknown value ' | v_value);
END
@[EMAIL PROTECTED]
 it possible to set the MESSAGE_TEXT dynamically?

Regards,
Serman D.
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