warning about casemap on inspircd network

```
WARN -- irc.rb:134:in `must_be': Casemap mismatch (#<Irc::RfcCasemap:0x00005569e96efd68 "A-Z{-~" ~(rfc1459)~ "a-z[-^"> != #<Irc::AsciiCasemap:0x00005569e96ece10 "A-Z" ~(ascii)~ "a-z">)
```

This is on a network running inspircd.
What causes this message?
How to prevent it?
Is that something that can be fixed in rbot or is it inspircd’s fault?