  The Szego-Widom theorem provides an expression for the determinant of block
Toeplitz matrices in the asymptotic limit of large matrix dimension n. We show
that the presence of zero modes, i.e, eigenvalues that vanish as \alpha^n,
|\alpha|<1, when n \to \infty, require a modification of the Szego-Widom
theorem. A new asymptotic expression for the determinant of a certain class of
block Toeplitz matrices with one pair of zero modes is derived. The result is
inspired by 1-dimensional topological superconductors, and the relation with
the latter systems is discussed.
