diff --git a/src/ui/js/diff2html-ui-base.ts b/src/ui/js/diff2html-ui-base.ts
index 9b2317f..bfa68d5 100644
--- a/src/ui/js/diff2html-ui-base.ts
+++ b/src/ui/js/diff2html-ui-base.ts
@@ -151,13 +151,16 @@ export class Diff2HtmlUI {
         this.config.highlightLanguages = new Map(Object.entries(this.config.highlightLanguages));
       }
 
-      const hljsLanguage =
+      let hljsLanguage =
         language && this.config.highlightLanguages.has(language)
           ? // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
             this.config.highlightLanguages.get(language)!
           : language
           ? getLanguage(language)
           : 'plaintext';
+      if (language !== null && hljs.getLanguage(language) === undefined) {
+        hljsLanguage = 'plaintext';
+      }
 
       // Collect all the code lines and execute the highlight on them
       const codeLines = file.querySelectorAll('.d2h-code-line-ctn');