Can foreigners who only have income from part-time work apply for naturalization?

There are several conditions that must be met in order for a foreigner to be allowed to naturalize in Japan.
One of these is "being able to make a living through the assets or skills of oneself or that of a spouse or other relative with whom one shares the same household."
We are sometimes consulted by people who say, "I want to apply for naturalization because I have income that is the same as a salary from a part-time job." However, unfortunately, income from a part-time job is not considered to be "income sufficient to support the applicant."
This is because part-time workers have no income if they are not working, and therefore cannot be said to have a stable income, unlike company employees who earn a certain amount each month.
Of course, it's a different story if the applicant's spouse or relatives have enough income and assets to support the applicant.