  A simple model is presented that describes the free energy $W(J)$ of QCD
coupled to an external current that is a single plane wave, $J(x) = H \cos(k
\cdot x)$. The model satisfies a bound obtained previously on $W(J)$ that comes
from the Gribov horizon. If one uses this model to fit recent lattice data ---
which give for the gluon propagator $D(k)$ a non-zero value, $D(0) \neq 0$, at
$k = 0$ --- the data favor a non-analyticity in $W(J)$.
