diff --git a/src/SupabaseClient.ts b/src/SupabaseClient.ts
index ca5733cd..d54fadd8 100644
--- a/src/SupabaseClient.ts
+++ b/src/SupabaseClient.ts
@@ -315,7 +315,9 @@ export default class SupabaseClient<
       fetch,
       // auth checks if there is a custom authorizaiton header using this flag
       // so it knows whether to return an error when getUser is called with no session
-      hasCustomAuthorizationHeader: 'Authorization' in this.headers,
+      hasCustomAuthorizationHeader: Object.keys(this.headers).some(
+        (key) => key.toLowerCase() === 'authorization'
+      ),
     })
   }