diff --git a/Doc/libctb.tex b/Doc/libctb.tex
index 6d683987142..d15177ef1b9 100644
--- a/Doc/libctb.tex
+++ b/Doc/libctb.tex
@@ -1,5 +1,6 @@
 \section{Built-in module \sectcode{ctb}}
 \bimodindex{ctb}
+\renewcommand{\indexsubitem}{(in module ctb)}
 
 This module provides a partial interface to the Macintosh
 Communications Toolbox. Currently, only Connection Manager tools are
diff --git a/Doc/mac/libctb.tex b/Doc/mac/libctb.tex
index 6d683987142..d15177ef1b9 100644
--- a/Doc/mac/libctb.tex
+++ b/Doc/mac/libctb.tex
@@ -1,5 +1,6 @@
 \section{Built-in module \sectcode{ctb}}
 \bimodindex{ctb}
+\renewcommand{\indexsubitem}{(in module ctb)}
 
 This module provides a partial interface to the Macintosh
 Communications Toolbox. Currently, only Connection Manager tools are