Triple-Modular Redundancy (TMR) is the simplest and most effective fault tolerant design method for ICs, where three copies of a given circuit are employed and a majority voter produces the voted output. Asynchronous circuits, however, exhibit various characteristics that limit the applicability of TMR on such designs. Specifically, the difficulty stems from the fact that asynchronous circuits communicate with their environment through hazard-free output transitions. This hazard-free property needs to be preserved when hardware providing fault tolerance is added. Therefore, performing TMR-based fault tolerance in asynchronous circuits requires the development of a hazard-free majority voter. In this work, we first demonstrate how and why a typical majority voter design would fail to preserve the hazard-free property of its response when a transient error occurs at one of its inputs. We then propose a novel hazard-free majority voter design for the TMR-based fault tolerant method. Finally, using PSpice simulations, we verify the effectiveness of the proposed voter design in preserving the hazard-free property of the response of an asynchronous circuit, and we assess its area overhead over that of a typical majority voter.