Topological invariants of electron wavefunctions in condensed matter reveal many intriguing phenomena(1,2). A notable example is provided by topological insulators, which are characterized by an insulating bulk coexisting with a metallic boundary state(3,4). Although there has been intense interest in Bi-based topological insulators(5,6), their behaviour is complicated by the presence of a considerable residual bulk conductivity(7-10). Theories predict(11,12) that the Kondo insulator system SmB6, which is known to undergo a transition from a Kondo lattice metal to a small-gap insulator state with decreasing temperature, could be a topological insulator. Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements(13-15), these have not demonstrated the topologically protected nature of the metallic surface state. Here we report thickness-dependent transport measurements on doped SmB6, and show that magnetic and non-magnetic doping results in contrasting behaviour that supports the conclusion that SmB6 shows virtually no residual bulk conductivity.