Opinion by
Judge Craig,In this appeal by a foreign corporation from a denial of a reassessment petition by the Board of Finance and Revenue, the question of law is whether the Department of Revenue must mail a notice of sales and use tax assessment to the address listed on a foreign corporations registry statement1 to apprise the corporation of the assessment.
Evergreen Helicopters (Evergreen) is an Oregon corporation which dispatches helicopters, for construction and crop dusting, from an office it maintains in Pennsylvania. Evergreen filed a registry statement with the Pennsylvania Department of State which lists its Oregon headquarters as the place to which our Department of Revenue should direct correspondence. The Department of Revenue sent a notice of assessment of sales and use taxes to Evergreens Pennsylvania office only. That office forwarded the notice to the corporations director of taxation in Oregon. Evergreen did not immediately petition for a reassessment, however, because its director of taxation suffered a breakdown and absconded with the departments notice of assessment. When Evergreen finally became aware of the assessment, the prescribed period for filing a petition for reassessment with the Board of Appeals had lapsed.2 Consequently, the Board of Appeals denied Evergreen’s appeal; the Board of Finance and Revenue affirmed.
*352Evergreen seeks to vacate the Board of Finance and Revenues order and remand the case for a hearing on the merits of the assessment on the ground of a denial of due process because the department failed to send the notice of assessment directly to Evergreens Oregon office. We must vacate the order of the Board of Finance and Revenue and remand for a hearing on the merits of the assessment.
By filing a registry statement with the Department of State under section 206(B) of the Business Corporation Law, 15 P.S. §1206(B), Evergreen designated where it wanted notices of tax assessments to be sent. Section 1206(B) states:
The registry statement . . . shall set forth:
(2) The post office address to which the corporation desires correspondence to be directed by the Department of Revenue. . . .
The crucial determinant in this case is one of the Department of Revenues own regulations, in effect when the notice of assessment was served, which required the department to direct such notices, not only to a known business address of the taxpayer in Pennsylvania, but also to such address as Evergreen had designated as one “desired” for notices—as it did in its registry statement. Under the applicable regulation, proper service of notices is stated to be a prerequisite to the imposition of liability on the taxpayer. The provisions of the departments regulation governing the service of such notices, 61 Pa. Code §35.1(d)(3),3 then read:
(3) Service. Service of notice shall conform with the following:
(i) Except as provided in this paragraph, as a prerequisite to a valid determination of personal *353liability, notice shall be served upon the person assessed by sending a copy of the notice by first-class mail to him at the following places:
(A) At his last known business or home post office address.
(B) At such address as the person may have designated, in writing, as the address at which he desired notice to be sent. (Emphasis added.)
The regulation hence requires the department to mail the essential notice to the described “places,” signifying by the use of the plural form that the listing is conjunctive and cumulative. Thus, because Evergreen had designated its Oregon headquarters in its registry statement as the address it “desires” to be used for revenue correspondence, the department had to mail notices to that office, as well as to the mailing address in Pennsylvania. The department could have written its own regulation to use the word “or,” in order to state the listing disjunctively. It did not. The department could have written the regulation to refer to “either of the following places.” It did not. Because the department did not follow the manner of service it prescribed for itself, we must conclude that Evergreens tax liability was not validly determined.
The department seeks to rely on the feet that Evergreen, on its application for a sales and use tax license, had entered its Pennsylvania office address as its “mailing address.” However, nothing in section 208 of the Tax Reform Code of 1971,4 concerning that application, negates the cumulative requirement of the regulation by any indication that such mailing address is to be the sole address where notices are to be sent. Considered together with section 208, the license application obviously calls for a mailing address to deal with the *354situation where an applicant has more than one place of business in Pennsylvania. The mailing address entered on the application therefore indicates which of those several addresses is to be used as the address of the principal place of business in Pennsylvania. However, at least during the time period involved here, there was nothing in the law or regulations eliminating the additional requirement that the department send notice also to an address designated as the desired one for such notices.
We note that the departments foilure to send notice directly to Evergreens Oregon office was actually a harmless error because Evergreens lack of actual notice resulted from the action of one of its own employees, not from the departments misdirected service. Nevertheless, an administrative agency’s regulations which have been duly authorized and promulgated have the force of law and are binding upon that agency. Fumo v. Department of Insurance, 58 Pa. Commonwealth Ct. 392, 427 A.2d 1259 (1981)., Consequently, because the department did not fulfill its own prerequisite to liability, the Board of Finance and Revenue’s order affirming the denial of Evergreen’s appeal is vacated and the case remanded for a hearing on Evergreen’s sales and use tax assessment.
Order
Now, October 20, 1986, the order of the Board of Finance and Revenue, at Docket No. RST-7654 402384 SUT AM-77825 S26, dated October 30, 1984, is vacated and the case remanded for a hearing on the petitioner’s sales and use tax assessment.
Jurisdiction relinquished.
A registry statement is a document which a foreign corporation is required to file with the Department of State under section 1004 of the Business Corporation Law, Act of May 5, 1933, P.L. 364, as amended, 15 P.S. §2004.
Section 232 of the Tax Reform Code of 1971, 72 P.S. §7232, provides that the taxpayer has thirty days after the notice of assessment is mailed in which to file a notice of intention to petition for reassessment with the Department of Revenue.
The 1979 version of section 35.1 was amended by 15 Pa. B. 3651 (1985).
Act of March 4, 1971, P.L. 6, as amended, 72 P.S. §7208.