Clarifying why EXECUTE AS USER / LOGIN does not return the expected results?

I am running the following query against a database:

execute as user = 'domain\username'
select * from fn_my_permissions(null, 'DATABASE')
order by subentity_name, permission_name
revert;

But the following error gets thrown:

Cannot execute as the database principal because the principal "dev\spadmin" does not exist, this type of principal cannot be impersonated, or you do not have permission.

The user is the dbo of the database, and when I open up the properties in management studio, I can see that it is associated with that login. Running EXECUTE AS LOGIN = 'domain\username' does return results, on the other hand. And if I explicitly run EXECUTE AS USER = 'dbo', I get results. I also have a different database where this same scenario returns results with both EXECUTE AS USER and EXECUTE AS LOGIN.

In another scenario with a different user, I have ran EXECUTE AS LOGIN = 'domain\username' and I do not get results, but I do get results with EXECUTE AS USER = 'domain\username'.

Both users in these scenarios are associated with logins that are members of db_ownerfor the database.

Can anybody tell me why these queries are not returning the results that I expect? And let me know if I am missing any important information. Thanks!

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The problem is that because the Login domain\username is the dbo of the database, that also means that the name of their corresponding User within that database is dbo and not domain\username.