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Impersonate user or login mapped to certificate

Assume that there is a certificate created in database

create certificate certName
    with subject = 'subj';

And a user mapped to this certificate

create user userName
    from certificate certName;

Trying to impersonate this user directly

execute as user = 'userName';

or specifying user in the execute as clause of the module

create procedure procName
with execute as 'userName'
    set nocount on;

returns error

Msg 15517, Level 16, State 1 ...
Cannot execute as the database principal because the principal "userName" does not exist, this type of principal cannot be impersonated, or you do not have permission.

However, I was not able to find this restriction mentioned in documentation (here and here), where the only relevant statement seems is

user_name must exist in the current database and must be a singleton account. user_name cannot be a group, role, certificate, key, or built-in account, such as NT AUTHORITY\LocalService, NT AUTHORITY\NetworkService, or NT AUTHORITY\LocalSystem.

Is it possible to impersonate user (or login) mapped to certificate or not?