diff --git a/plugins/tiddlywiki/highlight/highlightblock.js b/plugins/tiddlywiki/highlight/highlightblock.js
index e974193de..9fb0c3d2a 100644
--- a/plugins/tiddlywiki/highlight/highlightblock.js
+++ b/plugins/tiddlywiki/highlight/highlightblock.js
@@ -17,7 +17,7 @@ var CodeBlockWidget = require("$:/core/modules/widgets/codeblock.js").codeblock;
 CodeBlockWidget.prototype.postRender = function() {
 	var domNode = this.domNodes[0];
 	if($tw.browser && this.document !== $tw.fakeDocument && this.language) {
-		domNode.className = this.language;
+		domNode.className = this.language.toLowerCase();
 		var hljs = require("$:/plugins/tiddlywiki/highlight/highlight.js").hljs;
 		hljs.tabReplace = "    ";
 		hljs.highlightBlock(domNode);