clsql query does not work in some cases

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When searching character column with integer value it returns nil

For example, column contains "31159" value

(find-persistent-objects *default-store* 
                           'citilink-excel-data-item 
                           :where [= [article] "31159"])

returns nil
and

(find-persistent-objects *default-store* 
                           'citilink-excel-data-item 
                           :where [= [article] 31159])

returns record

It seems that this is clsql issue