diff --git a/doc/whatis.texi b/doc/whatis.texi
index 8c1f45ddb..891e27552 100644
--- a/doc/whatis.texi
+++ b/doc/whatis.texi
@@ -35,10 +35,11 @@ services can authenticate each other.
 @end macro
 @end ifinfo
 
-@tex
-@def@xsub#1{$_{#1}$}
-@global@let@sub=@xsub
-@end tex
+@iftex
+@macro sub{arg}
+@textsubscript{\arg\}
+@end macro
+@end iftex
 
 @ifhtml
 @macro sub{arg}