Attribute values of object types cannot be modified

I created a point
`SELECT *
FROM ag_catalog.cypher('zjdata_graph', $$
CREATE (a:a {id:1,map:{name:'a'}})
RETURN id(a), properties(a)
$$) as (id ag_catalog.agtype,properties ag_catalog.agtype)`

results are as follows

I modify the map property value
`SELECT *
FROM ag_catalog.cypher('zjdata_graph', $$
MATCH (a:a)
WHERE a.id = 1
SET a.map = '{"name":"b"}'
RETURN id(a), properties(a)
$$) as (id ag_catalog.agtype,properties ag_catalog.agtype)`

This is not the result I want, please tell me how to solve it