The problem of electromagnetic scattering by homogeneous spheres with relative impedance equal to unity is considered. From the Mie series expression for the induced internal fields, the details of the condition responsible for producing a null in the backscattering cross section are deduced. The existence of this null is attributable solely to destructive interference, regardless of frequency, and is valid for either lossy or lossless media. The returned signal in the vicinity of the null is determined from numerical computation of the bistatic cross sections.