A request for PMC2096233 to the citoid service API adds null as a value for PMID field. (i.e. http://localhost:1970/api?format=mediawiki&search=PMC2096233 ). This is because of the function addPubMedIdentifiers in /lib/Exporter.js extracts the pubmed from Zotero's "extra" field and Zotero returns null as the PMID. We should do basic validation of this field so that 'null' is not added as a PMID to the citation.
In order to get citoid working on your computer, you'll need to download both Node version 10.0 (for citoid) and xpcshell version 29.0 (for Zotero) to get both of them working. There are installation instructions and more information available at https://www.mediawiki.org/wiki/Citoid