Android : Update singel cell of a row in an SQLite database on Android

I wont to update a single cell of a row in the database. However the row contains of 5 columns so and i would like to not passing all the other values as well as they should remain the same.

I have this code snippet:

Cursor cursor = db.query(STATION_TABLE, null, null, null, null, null, null);

        //If the database already include some stations.
        if(cursor.moveToFirst())
        {
            ContentValues stationValues = new ContentValues();

            for(StopLocation station: stations)
            {
                stationValues.clear();

                //The database already includes the station
                if(cursor.getString(POS_STA_ID).equals(station.getId()))
                {
                    values.put(KEY_STA_DISTANCE, "null");

                    db.update(STATION_TABLE, stationValues, KEY_STA_ID + "=?", new String[]{station.getId()});
                }

The db.update method throws this exception:

java.lang.IllegalArgumentException: Empty values

Any ideas on how to solve this?