According to the PDF specification (Adobe® Portable Document Format Version 1.7):
Append a rectangle to the current path as a complete subpath, with lower-left corner (x, y) and dimensions width and height in user space.

The operation: 

x y width height re

An invalid case:

0 - 595 -842 re

This leads to an exception thrown: System.NullReferenceException: 'Object reference not set to an instance of an object.'