Queryvalue Limit (?)

Does Ax2009 has a limit for queryvalue?

Sample code:

    PurchLine   purchLine;
    str         queryValue;
    ;
    while select PurchId from purchLine
       group by PurchId
       where purchLine.FIELD1 like _projId
          || purchLine.FIELD2 like _projId
    {
        queryValue = queryValue + purchLine.PurchId + ',';
    }
    return queryValue;

on return I have an error.

Tnanks