diff --git a/src/core/parentClient.ts b/src/core/parentClient.ts
index 3a6ab65..b20882d 100644
--- a/src/core/parentClient.ts
+++ b/src/core/parentClient.ts
@@ -41,7 +41,7 @@ export class ParentClient extends BaseClient {
       headers: headers,
       credentials: undefined,
     });
-    if (response.status != 302 || !response.headers.get("set-cookie")) {
+    if (response.status != 302 || !response.headers.has("set-cookie")) {
       throw new Error(
         "Unauthenticated: ClassCharts returned an error: " +
           response.status +
diff --git a/src/core/studentClient.ts b/src/core/studentClient.ts
index 000c857..464a22d 100644
--- a/src/core/studentClient.ts
+++ b/src/core/studentClient.ts
@@ -41,7 +41,7 @@ export class StudentClient extends BaseClient {
       body: formData,
       redirect: "manual",
     });
-    if (request.status != 302 || !request.headers.get("set-cookie")) {
+    if (request.status != 302 || !request.headers.has("set-cookie")) {
       await request.body?.cancel(); // Make deno tests happy by closing the body, unsure whether this is needed for the actual library
       throw new Error(
         "Unauthenticated: ClassCharts didn't return authentication cookies",