Adding a private key tries to validate as Bitcoin address

[ripper234]

When adding a private key, the page insists the value should be a Bitcoin address when pasting a BASE58 private key.

[ThorvaldAagaard]

I had the same problem, and figured out it was because I was pasting a private key from Armory, and that private key contained spaces. So just remove spaces from the private key and try again. Ofc. it would be nice with a better error message