Puncture-resistant device for a non-pneumatic mounted assembly of a two-wheeled vehicle, in particular of a bicycle, this device comprising an annular support which is configured to be mounted between a rim and a tire of this mounted assembly so as to permanently support this tire and which comprises a crosslinked rubber composition of cellular type with closed cells, said composition being based on at least one elastomer and comprising a reinforcing inorganic filler and at least one organic blowing agent, characterized in that said composition comprises at least one active grade zinc oxide capable of activating the thermal decomposition of said blowing agent so as to stabilize the expansion of the support by giving it a minimized average density.