diff --git a/chapters/synchronous.tex b/chapters/synchronous.tex
index fb7977b79..03b38e746 100644
--- a/chapters/synchronous.tex
+++ b/chapters/synchronous.tex
@@ -522,7 +522,7 @@ \section{Clock Constructors}\label{clock-constructors}
 
 \section{Clocked State Variables}\label{clocked-state-variables}
 
-\begin{definition}[Clocked state variable]\label{def:clocked-state-variable}\index[clocked!state variable]\index{state variable!clocked}
+\begin{definition}[Clocked state variable]\label{def:clocked-state-variable}\index{clocked!state variable}\index{state variable!clocked}
 A component expression which is not a parameter, and to which \lstinline!previous! has been applied.
 \end{definition}