diff --git a/index.js b/index.js
index 2dc2346..95ab97e 100644
--- a/index.js
+++ b/index.js
@@ -476,7 +476,7 @@ Each terminal symbol is translated into a corresponding mathml node.*/
 var AMnestingDepth, AMpreviousSymbol, AMcurrentSymbol;
 
 function AMTgetTeXsymbol(symb) {
-  let pre = '';
+  var pre = '';
   if (typeof symb.val == "boolean" && symb.val) {
     pre = '';
   } else {