diff --git a/plugins/custom-class/prism-custom-class.js b/plugins/custom-class/prism-custom-class.js
index e12f8477d1..bb5bd6a6eb 100644
--- a/plugins/custom-class/prism-custom-class.js
+++ b/plugins/custom-class/prism-custom-class.js
@@ -7,7 +7,9 @@ if (
 	return;
 }
 
-var options = {};
+var options = {
+	classMap: {}
+};
 Prism.plugins.customClass = {
 	map: function map(cm) {
 		options.classMap = cm;
diff --git a/plugins/custom-class/prism-custom-class.min.js b/plugins/custom-class/prism-custom-class.min.js
index 03d34f5b09..f22063f557 100644
--- a/plugins/custom-class/prism-custom-class.min.js
+++ b/plugins/custom-class/prism-custom-class.min.js
@@ -1 +1 @@
-!function(){if("undefined"!=typeof self&&self.Prism||"undefined"!=typeof global&&global.Prism){var s={};Prism.plugins.customClass={map:function(i){s.classMap=i},prefix:function(i){s.prefixString=i}},Prism.hooks.add("wrap",function(i){(s.classMap||s.prefixString)&&(i.classes=i.classes.map(function(i){return(s.prefixString||"")+(s.classMap[i]||i)}))})}}();
\ No newline at end of file
+!function(){if("undefined"!=typeof self&&self.Prism||"undefined"!=typeof global&&global.Prism){var s={classMap:{}};Prism.plugins.customClass={map:function(i){s.classMap=i},prefix:function(i){s.prefixString=i}},Prism.hooks.add("wrap",function(i){(s.classMap||s.prefixString)&&(i.classes=i.classes.map(function(i){return(s.prefixString||"")+(s.classMap[i]||i)}))})}}();
\ No newline at end of file