Another generalization hierarchy problem

[abrea]

Dear list:
I am preparing an entity relationship diagram and encountered the 
following problem:
The entity "individual" has the attributes firstname, lastname.
The entity "company" has the attributes name, companytype.
Now I wish to include the subtype "customer" with the attributes 
taxid, billingaddress. However, a customer may be either an 
individual or a company, and I understand that a subtype can only 
have one supertype.
(I cannot put "customer" as the supertype because there are many 
individuals and companies that are not customers.)

How can I implement the subentity "customer" in my ERD?
Can I do it at all?
Thank you

Alberto Brea