Note that Equation 17-16 has the expression bD. Since MM and Hamada assumed that corporate debt is riskless, its beta should be zero. However, if corporate debt is not riskless, then its beta, bD, may not be zero. Assuming bonds lie on the Security Market Line, a bond’s required return, rd, can be expressed as rd = rRF + bDRPM. Solving for bD gives bD = (rd – rRF)/RPM.