Selecting multiple collections on the advanced search page?

We are working on a custom theme for our Omeka archive. In the theme is a customized version of the items/advanced-search page. We edited the advanced-search page to allow multiple ('multiple' => 'true',) for the collections list. However, when we actually select multiple collections and then try to search (search button), we received the following error:


Mysqli prepare error: Operand should contain 1 column(s)

Would anyone happen to have any suggestions as to how to go about troubleshooting and / or fixing search functionality to allow for multiple collection IDs?

We are trying to achieve a similar goal.

With the Collection Tree plugin installed, we want to add the option to search within a parent collection as well as all of its child collections. However the advanced search only allows one collection per search.

Did you find a solution to this problem?