[Exist-open] Problem with aggregate function, result xdt:untypedAtomic

[xx r. <xx...@wi...>]

Hi to you all,

there have been a lot of times that you helped me understand xquery and solve numerous problems, so I'm comming back with another one.
I want to find the average level per competence_category for a certain trainer, so I used a join and an aggregate function, but It seems that is not ok.

Query
for $comp in distinct-values(doc("/db/CMS_Project/Competences.xml")/Competences/Competence/Competence_Category),

$z in doc("/db/CMS_Project/trainers.xml")/Trainers/TrainerInformation/Competences

let $a := doc("/db/CMS_Project/trainers.xml")/Trainers/TrainerInformation/Competences[

    Competence_Level ne "-" and Competence_Name=$z/Competence_Name and ../Identification/Name="ioannidis"]

where $comp/../Competence_Name = $z/Competence_Name

return <tr style="background-color:#EFECC5"><td>{$comp}</td>

<td align="center">{round-half-to-even(avg($a/Competence_Level),2)}</td></tr>


Result

An exception occurred during query execution: cannot convert xdt:untypedAtomic('Professional') to a node set

Can I have AGAIN a little help?

Thank you very much


Regards
Christos