diff --git a/src/resend.ts b/src/resend.ts
index bc2902ba..5e1446d0 100644
--- a/src/resend.ts
+++ b/src/resend.ts
@@ -7,8 +7,16 @@ import { Emails } from './emails/emails';
 import { isResendErrorResponse } from './guards';
 import { ErrorResponse } from './interfaces';
 
-const baseUrl = process.env.RESEND_BASE_URL || 'https://api.resend.com';
-const userAgent = process.env.RESEND_USER_AGENT || `resend-node:${version}`;
+const defaultBaseUrl = 'https://api.resend.com';
+const defaultUserAgent = `resend-node:${version}`;
+const baseUrl =
+  typeof process !== 'undefined' && process.env
+    ? process.env.RESEND_BASE_URL || defaultBaseUrl
+    : defaultBaseUrl;
+const userAgent =
+  typeof process !== 'undefined' && process.env
+    ? process.env.RESEND_USER_AGENT || defaultUserAgent
+    : defaultUserAgent;
 
 export class Resend {
   private readonly headers: Headers;
@@ -20,7 +28,9 @@ export class Resend {
 
   constructor(readonly key?: string) {
     if (!key) {
-      this.key = process.env.RESEND_API_KEY;
+      if (typeof process !== 'undefined' && process.env) {
+        this.key = process.env.RESEND_API_KEY;
+      }
 
       if (!this.key) {
         throw new Error(