—Order, Supreme Court, Bronx County (Douglas McKeon, J.), entered April 19, 1993, *96which granted defendants’ motion to dismiss the complaint for failure to serve a notice of claim upon defendant New York City Health and Hospitals Corporation (HHC), and order, same court and Justice, entered January 7, 1994, which, inter alia, denied plaintiff’s motion for renewal, unanimously affirmed, without costs.
Defendants were under no duty to raise plaintiff’s failure to serve a notice of claim on defendant HHC as an affirmative defense, or otherwise bring to plaintiff’s attention that defendant City of New York, on whom plaintiff did serve a notice of claim, was not a proper party defendant (see, Ceely v New York City Health & Hosps. Corp., 162 AD2d 492, 493). Plaintiff’s attorney should have known that a notice of claim had to be served on HHC. Defendant’s conduct in not affirmatively advising plaintiff, the appearance of an HHC attorney at the General Municipal Law § 50-h hearing, and defendants’ joint participation in the litigation for years before moving to dismiss for failure to serve a notice of claim on the proper party does not give rise to an estoppel claim (see, Matter of Rieara v City of N. Y. Dept. of Parks & Recreation, 156 AD2d 206, 207). Further, "[T]he mere fact” that both the City and HHC are represented by the Corporation Counsel "does not provide the necessary nexus to equate service of a notice of claim on one with service on the other” (Ceely v New York City Health & Hosps. Corp., supra, at 493). This is not altered by the fact noted above that the City conducted a hearing pursuant to General Municipal Law § 50-h (supra), at which plaintiff was examined by an HHC attorney (Adams v New York City Tr. Auth., 140 AD2d 572). Nor does General Municipal Law § 50-e (3) (c) avail plaintiff. "While a municipal corporation may, by its conduct, waive an irregularity in the notice of claim, the requirements as to the manner or time of service may not be so waived” (Adams v New York City Tr. Auth., supra, at 573). Failure to serve a necessary party is not a mere irregularity.
In any event, the failure to diagnose a medical condition does not constitute continuous treatment (see, McDermott v Torre, 56 NY2d 399). Concur—Rosenberger, J. P., Kupferman, Asch and Tom, JJ.