missing FROM-clause entry for table

Hi group,

I get a mysterious errormessage while executing the following query:
(I left the tabledefinitions out, since I think they don't matter. If I
am wrong I'll be happy to post them.)

SELECT DRV1.languageid, DRV1.categorypropertyid,
CPL.categorypropertylangname
FROM (SELECT L.languageid, CP.categorypropertyid FROM tbllanguage AS L,
tblcategoryproperty AS CP) AS DRV1
LEFT OUTER JOIN tblcategorypropertylang AS CPL ON
((CLP.languageid=DRV1.languageid) AND
(CPL.categorypropertyid=DRV1.categorypropertyid));

ERROR:  missing FROM-clause entry for table "clp"

While this one is working just fine:

SELECT DRV1.languageid, DRV1.categorypropertyid, categorypropertylangname
    FROM (SELECT L.languageid, CP.categorypropertyid FROM tbllanguage AS
L, tblcategoryproperty AS CP) AS DRV1
    LEFT OUTER JOIN tblcategorypropertylang ON
((tblcategorypropertylang.languageid=DRV1.languageid) AND
(tblcategorypropertylang.categorypropertyid=DRV1.categorypropertyid));

The only difference is that I didn't use an alias CPL for
tblcategorypropertylang.
(The above queries are dynamically created by PHP based on a lot of
other, not related, things, and I would like to use the first way
because that makes sense in my current project.)

I Googled a little for the errormessage, but to no avail.
The second query is also good enough for me, but I am curious what I am
missing here.

What is causing this?

Thanks for your time.

Regards,
Erwin Moller