diff --git a/CHANGELOG.md b/CHANGELOG.md
index 697e82468..68d313630 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [12.0.4] - WIP
 ### Fixed
 - Fix crash introduced in `12.0.3` when processing strikethrough (`~~`) and similar plugins, #742.
+- Avoid fenced token mutation, #745.
 
 
 ## [12.0.3] - 2020-12-07
@@ -550,6 +551,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Renamed presets folder (configs -> presets).
 
 
+[12.0.4]: https://github.com/markdown-it/markdown-it/compare/12.0.3...12.0.4
 [12.0.3]: https://github.com/markdown-it/markdown-it/compare/12.0.2...12.0.3
 [12.0.2]: https://github.com/markdown-it/markdown-it/compare/12.0.1...12.0.2
 [12.0.1]: https://github.com/markdown-it/markdown-it/compare/12.0.0...12.0.1
diff --git a/lib/renderer.js b/lib/renderer.js
index d19b95449..5a173fe7d 100644
--- a/lib/renderer.js
+++ b/lib/renderer.js
@@ -60,7 +60,7 @@ default_rules.fence = function (tokens, idx, options, env, slf) {
   }
 
   // If language exists, inject class gently, without modifying original token.
-  // May be, one day we will add .clone() for token and simplify this part, but
+  // May be, one day we will add .deepClone() for token and simplify this part, but
   // now we prefer to keep things local.
   if (info) {
     i        = token.attrIndex('class');
@@ -69,6 +69,7 @@ default_rules.fence = function (tokens, idx, options, env, slf) {
     if (i < 0) {
       tmpAttrs.push([ 'class', options.langPrefix + langName ]);
     } else {
+      tmpAttrs[i] = tmpAttrs[i].slice();
       tmpAttrs[i][1] += ' ' + options.langPrefix + langName;
     }