Here is the steps to reproduce this issue:

Emacs version: 26.0.50 (from source code repo master branch)

Org-mode version: 9.0.8 (from source code repo master branch)

Here is the example link:

IRC: [[irc://irc.freenode.net/#nextcloud][#nextcloud]] on Freenode

I use Edebug to debug on function `org-irc-visit-erc`:

- [-] check out source code

- [X] org-open-at-point(nil)

- [X] org-irc-visit("//irc.freenode.net/#nextcloud")

- [ ] org-irc-visit-erc((("irc.freenode.net") "#nextcloud"))

- [ ] (port (or (string-to-number (cadr (pop link))) erc-default-port))

- [ ] (string-to-number (cadr (pop link)))

- [ ] (cadr (pop link)) -> (cadr ("irc.freenode.net")) -> nil

#+BEGIN_SRC emacs-lisp

(setq temp-link '(("irc.freenode.net") ("#clojure")))

(pop temp-link)

(cadr (pop temp-link))

#+END_SRC

- [ ] (string-to-number nil)

You can see what's wrong.

[stardiviner] <Hack this world!> GPG key ID: 47C32433
IRC(freeenode): stardiviner Twitter: @numbchild
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Blog: http://stardiviner.github.io/