Great question! Unfortunately, mapping "not set" or empty/null values in a Lookup Table is not supported by design.
Here's why your attempts don't work:
Empty string (""): The Lookup Table join key column requires that each value be unique with no falsy values. An empty string is considered a falsy value and is therefore not valid as a join key entry. [Create a Lookup Table]
(not set) and (empty): These are display labels that Mixpanel uses in its UI to represent undefined/null or empty string values — they are not actual property values that get stored or sent in your data. They cannot be used as literal join key values in a Lookup Table CSV. [Falsy Values]
In short, the Lookup Table join key column must contain unique, non-falsy values, so there is no supported way to map a row to events where the property is (not set), null, or an empty string.
As a best practice, Mixpanel recommends omitting properties entirely rather than sending null, empty string, or placeholder values like "N/A" — so ideally those cases wouldn't appear in your data at all.
