oracle error - ora-01427

my sql don't have subquery but i don't know why it will return error:  ora-01427 signle-row subquery retruns more than 1 rows

define as xmltype in oracle table column. 

<row id='1'><a1>name</a1><a2>address1</a2><a3>tel1</a3><a3 m='2'>20181231</a3><a3 m='3'>date3</a3></row>

my sql is as below:

select extractValue(xmltype,'/row/c1[position()=1]'), extract (xmltype,'/row/c3') from table_a;