We discuss the quantization of a system of slowly moving extreme Reissner-Nordstr\"om black holes. In the near-horizon limit, this system has been shown to possess an $SL(2,\mathbf{R})$ conformal symmetry. However, the Hamiltonian appears to have no well-defined ground state. This problem can be circumvented by a redefinition of the Hamiltonian due to de Alfaro, Fubini, and Furlan (DFF). We apply the Faddeev-Popov quantization procedure to show that the Hamiltonian with no ground state corresponds to a gauge in which there is an obstruction at the singularities of moduli space requiring a modification of the quantization rules. The redefinition of the Hamiltonian in the manner of DFF corresponds to a different choice of gauge. The latter is a good gauge leading to standard quantization rules. Thus the DFF trick is a consequence of a standard gauge-fixing procedure in the case of black hole scattering.