diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e41e8aef..155ea01f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,8 +12,8 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [18.x]
-        # See supported Node.js release schedule at https://nodejs.org/en/about/releases/
+        node-version: [18.x, 20.x]
+        # See supported Node.js release schedule at https://nodejs.org/en/about/previous-releases
 
     steps:
       - uses: actions/checkout@v3
diff --git a/README.md b/README.md
index 0afa9c5a..4d6e7280 100644
--- a/README.md
+++ b/README.md
@@ -132,8 +132,8 @@ such as injecting headers or adding log statements.
 
 ```js
 replicate.fetch = (url, options) => {
-  const headers = new Headers(options && options.headers);
-  headers.append("X-Custom-Header", "some value");
+  const headers = options && options.headers ? { ...options.headers } : {};
+  headers["X-Custom-Header"] = "some value";
 
   console.log("fetch", { url, ...options, headers });
 
diff --git a/biome.json b/biome.json
index 63919df3..807b9013 100644
--- a/biome.json
+++ b/biome.json
@@ -17,15 +17,16 @@
         "useMediaCaption": "off",
         "noSvgWithoutTitle": "off"
       },
+      "complexity": {
+        "useLiteralKeys": "off",
+        "useOptionalChain": "off"
+      },
       "performance": {
         "noAccumulatingSpread": "off"
       },
       "suspicious": {
         "noArrayIndexKey": "off",
         "noExplicitAny": "off"
-      },
-      "complexity": {
-        "useOptionalChain": "off"
       }
     }
   }
diff --git a/index.js b/index.js
index a3ed7f8f..ce407f91 100644
--- a/index.js
+++ b/index.js
@@ -191,15 +191,15 @@ class Replicate {
       url.searchParams.append(key, value);
     }
 
-    const headers = new Headers();
+    const headers = {};
     if (auth) {
-      headers.append("Authorization", `Token ${auth}`);
+      headers["Authorization"] = `Token ${auth}`;
     }
-    headers.append("Content-Type", "application/json");
-    headers.append("User-Agent", userAgent);
+    headers["Content-Type"] = "application/json";
+    headers["User-Agent"] = userAgent;
     if (options.headers) {
-      for (const [key, value] of options.headers.entries()) {
-        headers.append(key, value);
+      for (const [key, value] of Object.entries(options.headers)) {
+        headers[key] = value;
       }
     }