The application to stay was untimely, having been brought more than 20 days after the demand for arbitration (CPLR 7503 [c]). The court thus lacked the authority to address the issue of the arbitrator's jurisdiction ( Matter of Hartford Ins. Co. [Martin], 16 AD3d 149). The exception stated in Matter of Matarasso (Continental Cas. Co.) ( 56 NY2d 264) does not apply because it is undisputed that an arbitration agreement existed between the parties. In fact, it was petitioner who first compelled arbitration of the dispute; its current argument simply attacks the present viability of the contract containing the agreement to arbitrate ( see Matter of Fiveco, Inc. v Haber, 11 NY3d 140, 145).

Moreover, petitioner cannot avail itself of CPLR 7503 since it participated in the arbitration to the extent of attending a prehearing conference with the selected arbitrator, at which a hearing schedule and ground rules were decided upon, and even moved before the arbitrator to dismiss the proceeding on the ground that it was improperly brought ( see Matter of North Riv. Ins. Co. [Morgan], 291 AD2d 230, 233; Matter of Arner v Liberty Mut. Ins Co., 233 AD2d 321; Mufale v Romeo, 122 AD2d 591). Clearly, petitioner's attack on the propriety of the arbitrator's determination — on the ground that any defect in the original demand was rectified — constitutes an improper appeal of a nonfinal decision ( see Mobil Oil Indonesia v Asamera Oil [Indonesia], 43 NY2d 276, 281-282). Because petitioner failed to establish a likelihood of success on the merits, the court properly denied the motion for a preliminary injunction to stay arbitration, and dismissed the proceeding ( Mark Ross Co., Inc. v XE Capital Mgt., LLC, 46 AD3d 296).

We have considered petitioner's remaining arguments and find them unavailing.