diff --git a/src/core.ts b/src/core.ts
index 7b5a97bb..dfe9bd5b 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -1075,7 +1075,7 @@ function applyHeadersMut(targetHeaders: Headers, newHeaders: Headers): void {
 }
 
 export function debug(action: string, ...args: any[]) {
-  if (typeof process !== 'undefined' && process.env['DEBUG'] === 'true') {
+  if (typeof process !== 'undefined' && process?.env?.['DEBUG'] === 'true') {
     console.log(`Anthropic:DEBUG:${action}`, ...args);
   }
 }