Unable to reinstall Apple WWDR certificate

I was unable to compile XCode and realized I had two WWDR certificates — one in the "Login" keychain, one in the "Systems" keychain — and deleted both, planning to reinstall one into the "Systems" keychain.

When I download the WWDR from Apple and install it, it gives me this error:

An error occurred. Unable to import “Apple Worldwide Developer Relations Certification Authority”.
Error: 100013

Any thoughts, ideas?

Thanks,
Martin