Hugo Error Function "Union" not defined while trying to publish as subdomain branch on netlify

[ghost]

Not sure the cause, but any help would be appreciated. When trying to publish a hugo site as a subdomain on netlify, the branch fails with the following error:

7:07:46 PM: Executing user command: hugo
7:07:46 PM: Started building sites ...
7:07:46 PM: ERROR: 2019/01/24 00:07:46 template.go:477: template: theme/partials/menu.html:30: function "union" not defined
7:07:46 PM: ERROR: 2019/01/24 00:07:46 template.go:477: template: theme/partials/pagination.html:38: function "union" not defined
7:07:46 PM: ERROR: 2019/01/24 00:07:46 general.go:212: Error while rendering homepage: template: /opt/build/repo/themes/hugo-theme-techdoc/layouts/_default/baseof.html:2:20: executing "/opt/build/repo/themes/hugo-theme-techdoc/layouts/_default/baseof.html" at <.Page.Language>: can't evaluate field Page in type *hugolib.Node