Steps to reproduce:
1) Open a page that contains a link to an ISBN, in the form `[[Special:BookSources/9789041122735|9789041122735]]`. Click the link (in the link's context menu) to verify that the link can be opened in a new tab or window at https://en.wikipedia.org/wiki/Special:BookSources/9789041122735
2) Select and copy that link.
3) Create a new, non-template citation (Cite > Manual > Basic). Then paste the ISBN link into the basic citation dialog box..
4) The context menu for the ISBN link will be shown. Click the ISBN link in the link's context menu, to open the link in another window.
5) See error message from Safari 9: "There is no application set to open the URL special:BookSources/9789041122735." Alternatively, see the error message from Firefox 41: "The address wasn't understood Firefox doesn't know how to open this address, because one of the following protocols (special) isn't associated with any program or is not allowed in this context. You might need to install other software to open this address."
It seems to be trying to use `special:BookSources/9789041122735` as the URL, without any "https://xx.wikipedia.org" parts before it.
Reproducible in Safari 9 and Firefox 41.