diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/input.mjs b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/input.mjs
new file mode 100644
index 000000000000..2480aaca0c3b
--- /dev/null
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/input.mjs
@@ -0,0 +1,3 @@
+class B extends A {
+  b = 8;
+}
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/options.json b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/options.json
new file mode 100644
index 000000000000..35cf990f22a7
--- /dev/null
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/options.json
@@ -0,0 +1,14 @@
+{
+  "plugins": [
+    "proposal-class-properties",
+    [
+      "transform-runtime",
+      {
+        "corejs": { "version": 3, "proposals": true },
+        "helpers": true,
+        "regenerator": true
+      }
+    ]
+  ],
+  "presets": [["env", { "modules": false }]]
+}
diff --git a/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/output.mjs b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/output.mjs
new file mode 100644
index 000000000000..0db781ba8674
--- /dev/null
+++ b/packages/babel-plugin-transform-runtime/test/fixtures/regression/12863/output.mjs
@@ -0,0 +1,37 @@
+import _classCallCheck from "@babel/runtime-corejs3/helpers/classCallCheck";
+import _assertThisInitialized from "@babel/runtime-corejs3/helpers/assertThisInitialized";
+import _inherits from "@babel/runtime-corejs3/helpers/inherits";
+import _possibleConstructorReturn from "@babel/runtime-corejs3/helpers/possibleConstructorReturn";
+import _getPrototypeOf from "@babel/runtime-corejs3/helpers/getPrototypeOf";
+import _defineProperty from "@babel/runtime-corejs3/helpers/defineProperty";
+import _concatInstanceProperty from "@babel/runtime-corejs3/core-js/instance/concat";
+
+function _createSuper(Derived) { var hasNativeReflectConstruct = _isNativeReflectConstruct(); return function _createSuperInternal() { var Super = _getPrototypeOf(Derived), result; if (hasNativeReflectConstruct) { var NewTarget = _getPrototypeOf(this).constructor; result = _Reflect$construct(Super, arguments, NewTarget); } else { result = Super.apply(this, arguments); } return _possibleConstructorReturn(this, result); }; }
+
+function _isNativeReflectConstruct() { if (typeof Reflect === "undefined" || !_Reflect$construct) return false; if (_Reflect$construct.sham) return false; if (typeof Proxy === "function") return true; try { Boolean.prototype.valueOf.call(_Reflect$construct(Boolean, [], function () {})); return true; } catch (e) { return false; } }
+
+var B = /*#__PURE__*/function (_A) {
+  _inherits(B, _A);
+
+  var _super = _createSuper(B);
+
+  function B() {
+    var _context;
+
+    var _this;
+
+    _classCallCheck(this, B);
+
+    for (var _len = arguments.length, args = new Array(_len), _key = 0; _key < _len; _key++) {
+      args[_key] = arguments[_key];
+    }
+
+    _this = _super.call.apply(_super, _concatInstanceProperty(_context = [this]).call(_context, args));
+
+    _defineProperty(_assertThisInitialized(_this), "b", 8);
+
+    return _this;
+  }
+
+  return B;
+}(A);