attaching a contact to a "Lead"

Hi,

My code works fine when attaching contacts to customers but am receiving the following error when attempting to attach a contact to a Lead.

Invalid CUSTOMER reference key.

my code is:
                WriteResponse response;

                RecordRef contactRole = new RecordRef();
                contactRole.internalId = roleID;
                contactRole.type = RecordType.contactRole;
                contactRole.typeSpecified = true;

                RecordRef contactRef = new RecordRef();
                contactRef.internalId = _contactID;
                contactRef.type = RecordType.contact;
                contactRef.typeSpecified = true;

                RecordRef attachTo = new RecordRef();
                attachTo.internalId = companyID;
                attachTo.type = RecordType.customer;
                attachTo.typeSpecified = true;

                AttachContactReference attachReference = new AttachContactReference();
                attachReference.attachTo = attachTo;
                attachReference.contact = contactRef;
                attachReference.contactRole = contactRole;

You can attach contacts to leads on the UI so I would think you can do it through WS?