diff --git a/lectures/lecturecommon.sty b/lectures/lecturecommon.sty
index bdb18e4..8c614b9 100644
--- a/lectures/lecturecommon.sty
+++ b/lectures/lecturecommon.sty
@@ -1,4 +1,4 @@
-\def\filedate{9 February 2017}
+\def\filedate{26 September 2017}
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %
 % Common definitions shared across the various lecture document classes.
@@ -15,11 +15,12 @@
 % v1.2.1 Fixed fontspec option clash and made font handling consistent with XML framework.
 % v1.2.2 Fixed incorrect Roman font in math mode.
 % v1.2.3 Switched XeLaTeX fonts to new University standard (Minion Pro + Open Sans).
+% v1.2.4 Fixed "class number plus one" so that it can be passed as a number to other macros.
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 \NeedsTeXFormat{LaTeX2e}[2001/06/01]
-\ProvidesPackage{lecturecommon}[2017/02/09 INFO standard lecture setup v1.2.3]
+\ProvidesPackage{lecturecommon}[2017/09/26 INFO standard lecture setup v1.2.4]
 
 
 % Standard packages common to ALL lecture documents.
@@ -152,13 +153,16 @@
 
 
 % Class number.
+\newcounter{chapterplusone}
+\setcounter{chapterplusone}{\value{chapter}}
+\stepcounter{chapterplusone}
 \newcommand{\ClassNumber}{\thechapter}
-\newcommand{\ClassNumberPlusOne}{%
-    \addtocounter{chapter}{1}%
-    \thechapter%
-    \addtocounter{chapter}{-1}%
+\newcommand{\ClassNumberPlusOne}{\thechapterplusone}
+\newcommand{\SetClassNumber}[1]{%
+    \setcounter{chapter}{#1}%
+    \setcounter{chapterplusone}{#1}%
+    \stepcounter{chapterplusone}%
 }
-\newcommand{\SetClassNumber}[1]{\setcounter{chapter}{#1}}
 
 % Backwards compatibility with the old syntax
 \newcommand{\ChapterNumber}{\ClassNumber}