Fix exception when calculating fdh.
Fix exception when calculating fdh.

dinexcode/dinex-auth,xdv/ripple-authd,bankonme/ripple-authd,bankonme/ripple-authd,sentientwaffle/ripple-authd,ripple/ripple-authd,ripple/ripple-authd,42cc/ripple-authd,42cc/ripple-authd,dinexcode/dinex-auth,sentientwaffle/ripple-authd,xdv/ripple-authd