Error updating sql server geography type in trigger

Question

I have the following trigger in my database:

CREATE TRIGGER dbo.triggerGeocodedAddressUpdate ON dbo.Party AFTER UPDATE AS
    IF UPDATE(Latitude)
        UPDATE pt
        SET pt.GeocodedAddress = geography::Point(i.Latitude, i.Longitude, 4326)
        FROM dbo.Party AS pt INNER JOIN inserted AS i ON i.PartyId = pt.PartyId
        WHERE (i.Latitude IS NOT NULL AND i.Longitude IS NOT NULL);
        RETURN
    IF UPDATE(Longitude)
        UPDATE pt
        SET pt.GeocodedAddress = geography::Point(i.Latitude, i.Longitude, 4326)
        FROM dbo.Party AS pt INNER JOIN inserted AS i ON i.PartyId = pt.PartyId
        WHERE (i.Latitude IS NOT NULL AND i.Longitude IS NOT NULL);
        RETURN
GO

But every time I try to update a row in the Party table, I get an error, even if I don't change the Latitude or Longitude fields.

The error is:

geography::Point' failed because parameter 1 is not allowed to be null.

Any ideas? I'm pretty new to triggers, so I'm sorry if it's something simple and obvious.