diff --git a/source/lex.tex b/source/lex.tex
index 6f5bffac97..e5ecbd0901 100644
--- a/source/lex.tex
+++ b/source/lex.tex
@@ -177,6 +177,7 @@
 that common \grammarterm{encoding-prefix}.
 
 \item
+\indextext{concatenation!string}%
 Adjacent \grammarterm{string-literal} preprocessing tokens are concatenated\iref{lex.string}.
 
 \item