diff --git a/offlineimap/folder/Base.py b/offlineimap/folder/Base.py
index 8cc22df..b691c54 100644
--- a/offlineimap/folder/Base.py
+++ b/offlineimap/folder/Base.py
@@ -1092,9 +1092,10 @@ class BaseFolder(object):
             except OfflineImapError as e:
                 if e.severity > OfflineImapError.ERROR.FOLDER:
                     raise
-                self.ui.error(e, exc_info()[2])
+                self.ui.error(e, exc_info()[2], "while syncing %s [account %s]"%
+                                                (self, self.accountname))
             except Exception as e:
-                self.ui.error(e, exc_info()[2], "Syncing folder %s [acc: %s]"%
+                self.ui.error(e, exc_info()[2], "while syncing %s [account %s]"%
                                                 (self, self.accountname))
                 raise # Raise unknown Exceptions so we can fix them.
 
diff --git a/offlineimap/folder/IMAP.py b/offlineimap/folder/IMAP.py
index 4093fa0..edc2735 100644
--- a/offlineimap/folder/IMAP.py
+++ b/offlineimap/folder/IMAP.py
@@ -832,19 +832,22 @@ class IMAPFolder(BaseFolder):
             except imapobj.readonly:
                 self.ui.flagstoreadonly(self, uidlist, flags)
                 return
-            r = imapobj.uid('store',
+            response = imapobj.uid('store',
                 imaputil.uid_sequence(uidlist), operation + 'FLAGS',
                     imaputil.flagsmaildir2imap(flags))
-            assert r[0] == 'OK', 'Error with store: ' + '. '.join(r[1])
-            r = r[1]
+            if response[0] != 'OK':
+                raise OfflineImapError(
+                    'Error with store: %s'% '. '.join(response[1]),
+                    OfflineImapError.ERROR.MESSAGE)
+            response = response[1]
         finally:
             self.imapserver.releaseconnection(imapobj)
         # Some IMAP servers do not always return a result.  Therefore,
         # only update the ones that it talks about, and manually fix
         # the others.
         needupdate = list(uidlist)
-        for result in r:
-            if result == None:
+        for result in response:
+            if result is None:
                 # Compensate for servers that don't return anything from
                 # STORE.
                 continue