Fans who went to check out Primus' Sept. 13 and 14 shows in Chicago were treated to an extra little surprise. Due to drummer Tim 'Herb' Alexander's current health issues, Tool's Danny Carey has been filling in behind the kit, and thanks to his presence in the lineup, Primus blasted off into a cover of Tool's 'Ænema.'

Tim Alexander suffered a heart attack back in July, requiring the Primus / Puscifer drummer to undergo open heart surgery. Alexander's surgery was successful, with Primus' Les Claypool announcing that "the blockage from his heart has been removed with no complications and all signs point to a glorious and speedy recovery."

Alexander is still resting up, allowing Danny Carey to act as substitute for a handful of September tour dates. In the middle of Primus' two Chicago sets, the band jammed an interlude of 'Ænema.' Fan-filmed footage caught the Sept. 13 performance of 'Ænema,' during which ecstatic fans sang along with the Tool anthem.

Check out Primus' cover of 'Ænema' in the video above! To see Primus' 2014 U.S. tour dates, click here.