Index: Doc/library/subprocess.rst
===================================================================
--- Doc/library/subprocess.rst	(revision 71578)
+++ Doc/library/subprocess.rst	(working copy)
@@ -102,6 +102,13 @@
    variables for the new process; these are used instead of inheriting the current
    process' environment, which is the default behavior.
 
+   .. warning::
+
+      When replacing the environment you must provide any variables
+      required for the program to execute.  On Windows, in order to run
+      a side-by-side assembly the specified *env* must include a valid
+      :envvar:`SystemRoot`.
+
    If *universal_newlines* is :const:`True`, the file objects stdout and stderr are
    opened as text files, but lines may be terminated by any of ``'\n'``, the Unix
    end-of-line convention, ``'\r'``, the old Macintosh convention or ``'\r\n'``, the