Impersonation not fully functioning

I'm an admin and I'm having trouble with the impersonation feature for Problem Management. Right now, I tried using the impersonation feature and it works if I try to impersonate an 'Incident User' but when trying to impersonate an actual person (with a license), I get an error message stating "The user you selected could not be impersonated, typically this is because they had a blank login id." When trying to impersonate 'End User' I am automatically directed to our Employee Self Service page. Any thoughts as to why it's doing this?