When attempting to add an Itemstack from the Cursor to a Bundle with more than 64 items inside, the Player is kicked with a 

Internal Exception: io.netty.handler.codec.DecoderException: java.lang.IllegalArgumentException: item is null or air 

and the server creates a long list of netty io exceptions, related to CraftMetaBundle.

When instead having the Bundle on the Cursor and attempting to add an Itemstack by rightclicking on a slot, the action is simply denied without error.

This same sequence of events is also bugged in Vanilla, but instead of throwing an exception it adds an empty ItemStack of AIR to the bundle with every attempt.

[link Link|[MC-256599] Bundle adds AIR as a stack when attempting to add to a bundle with more than 64 items inside - Jira (mojang.com)]

(Confirmed on the Mojang bug tracker)