  We demonstrate the absence of a DC Stark shift in an ytterbium optical
lattice clock. Stray electric fields are suppressed through the introduction of
an in-vacuum Faraday shield. Still, the effectiveness of the shielding must be
experimentally assessed. Such diagnostics are accomplished by applying high
voltage to six electrodes, which are grounded in normal operation to form part
of the Faraday shield. Our measurements place a constraint on the DC Stark
shift at the $10^{-20}$ level, in units of the clock frequency. Moreover, we
discuss a potential source of error in strategies to precisely measure or
cancel non-zero DC Stark shifts, attributed to field gradients coupled with the
finite spatial extent of the lattice-trapped atoms. With this consideration, we
find that Faraday shielding, complemented with experimental validation,
provides both a practically appealing and effective solution to the problem of
DC Stark shifts in optical lattice clocks.
