"Field comparison" generates SQL that compares against the wrong field

I created a view of my host nodes with a reference to the registration settings. I added two fields: slots total and spaces used.

When I add a "Global: Fields Comparison (capacity_used < capacity_total)" filter, I get no results returned and the WHERE clause in the SQL Views generates looks like this:

WHERE (node.nid < registration_entity_node.capacity)

I'm not sure why node.nid is being used instead of the capacity used, but that's definitely wrong.