diff --git a/plugins/custom-class/prism-custom-class.js b/plugins/custom-class/prism-custom-class.js
index 24f56549e7..3eeb75f191 100644
--- a/plugins/custom-class/prism-custom-class.js
+++ b/plugins/custom-class/prism-custom-class.js
@@ -88,7 +88,7 @@
 		}
 
 		env.classes = env.classes.map(function (c) {
-			return prefixString + mapper ? mapper(c, env.language) : c;
+			return prefixString + (mapper ? mapper(c, env.language) : c);
 		});
 	});
 
diff --git a/plugins/custom-class/prism-custom-class.min.js b/plugins/custom-class/prism-custom-class.min.js
index da639b6707..e579293526 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 a,e,t="";Prism.plugins.customClass={add:function(n){a=n},map:function(s){e="function"==typeof s?s:function(n){return s[n]||n}},prefix:function(n){t=n||""}},Prism.hooks.add("wrap",function(s){if(a){var n=a({content:s.content,type:s.type,language:s.language});Array.isArray(n)?s.classes.push.apply(s.classes,n):n&&s.classes.push(n)}(e||t)&&(s.classes=s.classes.map(function(n){return t+e?e(n,s.language):n}))})}}();
\ No newline at end of file
+!function(){if("undefined"!=typeof self&&self.Prism||"undefined"!=typeof global&&global.Prism){var a,e,t="";Prism.plugins.customClass={add:function(n){a=n},map:function(s){e="function"==typeof s?s:function(n){return s[n]||n}},prefix:function(n){t=n||""}},Prism.hooks.add("wrap",function(s){if(a){var n=a({content:s.content,type:s.type,language:s.language});Array.isArray(n)?s.classes.push.apply(s.classes,n):n&&s.classes.push(n)}(e||t)&&(s.classes=s.classes.map(function(n){return t+(e?e(n,s.language):n)}))})}}();
\ No newline at end of file