Problem inviting my vassal to my alliance

[unmerged(35764)]

I'm playing England in very hard/normal. I have Eire and Scotland as vassals, and Eire is part of my alliance, along with a few other nations. Now, Eire and Scotland don't particularly like each other, they have had 3 wars in the last 30 years, and neither has gained anything.
I'm trying to get Scotland into my alliance so I can diploannex them later, and also so they don't get into a war with someone else in my alliance and force me to do something drastic. I have improved our relations to +200, and they keep rejecting my invitation. Why is that? Are there other prerequisites?

 

[Hallsten]

No, just continue trying. Sooner or later they'll agree...

 

[unmerged(36846)]

An alliance can only take 5 members.

 

[unmerged(9338)]

An alliance can only take 5 members.

Yes, that is my experience as well, so kick out your non-vassals

 

[unmerged(35764)]

Cool, thanks.

 

[ProfCC]

The AI can and often does have larger alliances, but the human player is limited to only 4 other alliance partners. Makes for some tough calls at times. Just last night, to get ready for my next move, I really needed France in my alliance, but I already had 4 partners. So, I had to ban one member to pick up the French. We'll see how that works out.