diff --git a/.gitignore b/.gitignore
index ba6052e..19b728d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 .idea
 package-lock.json
+yarn.lock
 dist/
 node_modules/
 lib/
diff --git a/package.json b/package.json
index 5b5124c..cee6114 100644
--- a/package.json
+++ b/package.json
@@ -5,7 +5,7 @@
   "scripts": {
     "clean": "rm -rf ./lib ./dist",
     "build": "webpack --mode production",
-    "compile": "tsc -p ./tsconfig.json",
+    "compile": "tsc && tsc -p ./tsconfig.esm.json",
     "test": "mocha --require ts-node/register 'test/**/*.spec.ts'",
     "lint": "eslint src --ext .ts,.tsx",
     "lint-fix": "eslint src --ext .ts,.tsx --fix"
@@ -14,9 +14,21 @@
   "license": "MIT",
   "main": "./lib/index.js",
   "exports": {
-    ".": "./lib/index.js",
-    "./sender": "./lib/sender.js",
-    "./listener": "./lib/listener.js"
+    ".": {
+      "types": "./lib/index.d.ts",
+      "import": "./lib/esm/index.js",
+      "default": "./lib/index.js"
+    },
+    "./sender": {
+      "types": "./lib/sender.d.ts",
+      "import": "./lib/esm/sender.js",
+      "default": "./lib/sender.js"
+    },
+    "./listener": {
+      "types": "./lib/listener.d.ts",
+      "import": "./lib/esm/listener.js",
+      "default": "./lib/listener.js"
+    }
   },
   "files": [
     "lib",
diff --git a/src/listener.ts b/src/listener.ts
index b5b404c..a8fa569 100644
--- a/src/listener.ts
+++ b/src/listener.ts
@@ -2,7 +2,7 @@ import {CALL_TYPE, RESPONSE_TYPE} from "./constants";
 import {MessageTarget, RPCRequest, WindowLike} from "./types";
 
 export default class Listener {
-    public fallbackSource: MessageTarget; // Only for testing
+    public fallbackSource?: MessageTarget; // Only for testing
 
     constructor(private readonly methods: {[fn: string]: (...args: any[]) => any}) {}
 
diff --git a/src/sender.ts b/src/sender.ts
index 39a4b2b..ee384f5 100644
--- a/src/sender.ts
+++ b/src/sender.ts
@@ -63,7 +63,7 @@ export function connect<T extends {[name: string]: (...args: any) => Promise<any
         const {id, error, result} = event.data;
         if (!promises.has(id)) throw new Error(`No promise waiting for call id ${id} on channel ${channel}`);
 
-        const {resolve, reject, timeoutId} = promises.get(id);
+        const {resolve, reject, timeoutId} = promises.get(id)!;
         promises.delete(id);
 
         if (timeoutId) parent.clearTimeout(timeoutId);
diff --git a/tsconfig.esm.json b/tsconfig.esm.json
new file mode 100644
index 0000000..f0f1e46
--- /dev/null
+++ b/tsconfig.esm.json
@@ -0,0 +1,14 @@
+{
+  "compilerOptions": {
+    "strict": true,
+    "baseUrl": "",    
+    "target": "es2020",
+    "module": "es6",
+    "moduleResolution": "node16",
+    "noUnusedLocals": true,
+    "sourceMap": true,
+    "rootDir": "src",
+    "outDir": "lib/esm"
+  },
+  "include": ["src"],
+}
diff --git a/webpack.config.js b/webpack.config.js
index 6018b65..cd7b019 100644
--- a/webpack.config.js
+++ b/webpack.config.js
@@ -2,7 +2,7 @@ const path = require("path");
 
 module.exports = {
     experiments: {
-        outputModule: true,
+        outputModule: false,
     },
     entry: {
       sender: "./src/sender.ts",
@@ -15,7 +15,7 @@ module.exports = {
         asyncChunks: false,
         library: {
             name: "simpleIframeRpc",
-            type: "amd"
+            type: "umd"
         }
     },
     module: {