Handle unset notBefore and notAfter values
diff --git a/doc/pyOpenSSL.tex b/doc/pyOpenSSL.tex
index ec8831d..5e4d531 100644
--- a/doc/pyOpenSSL.tex
+++ b/doc/pyOpenSSL.tex
@@ -297,6 +297,7 @@
                  YYYYMMDDhhmmss+hhmm
                  YYYYMMDDhhmmss-hhmm
 \end{verbatim}
+If no value exists for this field, \code{None} is returned.
 \end{methoddesc}
 
 \begin{methoddesc}[X509]{get_notAfter}{}
@@ -307,6 +308,7 @@
                  YYYYMMDDhhmmss+hhmm
                  YYYYMMDDhhmmss-hhmm
 \end{verbatim}
+If no value exists for this field, \code{None} is returned.
 \end{methoddesc}
 
 \begin{methoddesc}[X509]{set_notBefore}{when}