Why could `RowPitch` be impossibly large after a `Map` call?

Question

I am trying to save a screenshot.

First make a staging texture by copying the original texture's description with

CpuAccessFlags = CpuAccessFlags.Read,
Usage = ResourceUsage.Staging,
BindFlags = BindFlags.None,

and calling CopyResource

Next in a different thread I am calling Map (MapSubresource in SharpDX), and the call succeeds, however in the returned struct RowPitch has seemingly impossible value: 0xc204a428 (that's over 2GB).

Why am I getting an obviously invalid value?