Question : ERROR:  the text, netext, and image data types are invalid in this subquery or aggregate expression

I have a pretty simple update query as follows but I am getting the error that "The text, ntext, and image data types are invalid in this subquery or aggregate expression".  I have tried casting as nvarchar to now avail.  The data types of the source and target column are all ntext.  I have searched the internet high and low for a solution and have determined that this is an uncommon error but surely someone has encountered it before.  I suspect that it has to do with the way ntext, text are stored and that i need to do something special.  Just dunno.  Any thoughts?

UPDATE    Emails
SET TextContent = (SELECT     textcontent
                              FROM         emailtemplates
                              WHERE     emaillistid = 0),
HTMLContent = (SELECT     htmlcontent
                              FROM         emailtemplates
                              WHERE     emaillistid = 0)
WHERE     (EmailID = 1387)

Answer : ERROR:  the text, netext, and image data types are invalid in this subquery or aggregate expression

You'll have to use the UPDATETEXT statement

You'll find working exemples in BOL

Yet you could give a try to the following
if and only if there is
- only one record in Emails with id 1387
- only one record in EmailTemplates with id 0

UPDATE  a
SET TextContent = b.textcontent,
HTMLContent = b.htmlcontent
from   Emails a CROSS JOIN emailtemplates b
WHERE a.EmailID = 1387
and b.emaillistid = 0

HTH

Hilaire