diff --git a/moodle_emails/utils.py b/moodle_emails/utils.py
index 3f1cb1b..1d9f26b 100644
--- a/moodle_emails/utils.py
+++ b/moodle_emails/utils.py
@@ -1,3 +1,4 @@
+import json
 import re
 import pathlib
 from warnings import warn
@@ -74,15 +75,16 @@ class MailListDownloader():
     def course_list(self):
         userid, version = self.fch.fetch_userid_and_version()
         courses = self.fch.fetch_courses(userid)
-        return [ dict(id=c.id,cds=get_cds(c),fullname=c.fullname) for c in courses ]
+        return [ dict(cid=c.id,cds=get_cds(c),fullname=c.fullname) for c in courses ]
 
     def download_from_new_courses(self, path):
         path = pathlib.Path(path)
-        downloaded_file = path.with_suffix('.downloaded.npy')
+        downloaded_file = path.with_suffix('.downloaded.json')
         database_file = path.with_suffix('.db.hdf')
 
         if downloaded_file.is_file():
-            downloaded = list(np.fromfile(downloaded_file))
+            with downloaded_file.open() as f:
+                downloaded = json.load(f)
         else:
             downloaded = []
         
@@ -97,9 +99,9 @@ class MailListDownloader():
         
         for c in tqdm(self.course_list()):
             old_len = len(db)
-            if c['cds'] and not c['id'] in downloaded:
+            if c['cds'] and not c['cid'] in downloaded:
                 print(f"Downloading {c['fullname']}")
-                cid = c['id']
+                cid = c['cid']
                 cds = c['cds']
                 emails = self.course_emails(cid)
                 emails = pd.DataFrame(emails)
@@ -111,7 +113,8 @@ class MailListDownloader():
                 duplicates = len(emails) - new
                 print(f"\t{new_len-old_len} new, {duplicates} duplicates.")
         
-            np.array(downloaded).tofile(downloaded_file)
+            with downloaded_file.open('w') as f:
+                json.dump(downloaded, f)
             db.to_hdf(database_file, key='emails')
         return db