Problem inserting JSON with sub-select

I guess it's related to this topic 

So when I'm trying to add a new row using sub select like 

insert into <table> (text_field, json_field)
select distinct text, cast(json_value as json)
from <table2>
;

I see the error 

could not identify an equality operator for type json (449)

However, when I use explicit insert of VALUES (text, json), it works smoothly.

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