Question about Pope Benedict XVI

Throughout his pontificate, Pope Benedict XVI has focused each of his weekly addresses on a particular theme.

And Our Sunday Visitor has done a great job compiling each series of talks into attractive books:

But the newest collection of talks–and the one I’m most excited about–covers the Doctors of the Church. Benedict delivered one message per Doctor in the early months of this year. However, there was one surprising omission:

St. Peter Chrysologus.

Does anyone know why Pope Benedict skipped him?