Can't add my BA frequent flyer number to account: suggestions?

Hello everyone!
I just joined HHonors and I wanted to add my BA Executive Club number for double dipping ;)

But when I go into my profile, under Preferences and select British Airways Executive Club (Avios points) and add my number, I get this error

The member number entered is invalid for the selected partner


My membership number is 8 digits. I've tried adding 0s to make it 9 or 10 digits but still I get the same error.

Google didn't help as well... Do you have any hint?

Thanks a lot