diff --git a/.babelrc b/.babelrc
index 10a91da0e..37b74fe1c 100644
--- a/.babelrc
+++ b/.babelrc
@@ -1,5 +1,7 @@
 {
+  "plugins": ["@babel/proposal-class-properties"],
   "presets": [["@babel/env", {
+    "useBuiltIns": false,
     "targets": {
       "ie": "11"
     }
diff --git a/package.json b/package.json
index 6bb4b980c..94ae15c57 100644
--- a/package.json
+++ b/package.json
@@ -78,6 +78,8 @@
     ],
     "devDependencies": {
         "@babel/cli": "7.12.1",
+        "@babel/plugin-proposal-class-properties": "7.12.1",
+        "@babel/plugin-transform-classes": "7.12.1",
         "@babel/preset-env": "7.12.1",
         "@babel/types": "^7.12.6",
         "@cortex-js/prettier-config": "^1.0.0",
diff --git a/src/public/mathfield-element.ts b/src/public/mathfield-element.ts
index 4968d83ac..efad78d8f 100644
--- a/src/public/mathfield-element.ts
+++ b/src/public/mathfield-element.ts
@@ -1150,8 +1150,9 @@ declare global {
         MathfieldElement: typeof MathfieldElement;
     }
 }
-
-if (!window.customElements.get('math-field')) {
-    window.MathfieldElement = MathfieldElement;
-    window.customElements.define('math-field', MathfieldElement);
+if (typeof window != 'undefined' && window.customElements) {
+    if (!window.customElements.get('math-field')) {
+        window.MathfieldElement = MathfieldElement;
+        window.customElements.define('math-field', MathfieldElement);
+    }
 }