diff -r f243af7b5be3 -r 4c8280aaf6ad src/Pure/Thy/latex.ML
--- a/src/Pure/Thy/latex.ML	Sat Jan 06 21:25:16 2018 +0100
+++ b/src/Pure/Thy/latex.ML	Sun Jan 07 12:41:34 2018 +0100
@@ -76,7 +76,9 @@
 
 end;
 
-fun enclose_body bg en body = string bg :: body @ [string en];
+fun enclose_body bg en body =
+  (if bg = "" then [] else [string bg]) @ body @
+  (if en = "" then [] else [string en]);
 
 
 (* output name for LaTeX macros *)