diff --git a/offlineimap/folder/Base.py b/offlineimap/folder/Base.py
index 3913367..e3f863c 100644
--- a/offlineimap/folder/Base.py
+++ b/offlineimap/folder/Base.py
@@ -524,13 +524,16 @@ class BaseFolder:
 
         raise NotImplementedError
 
-    def savemessagelabels(self, uid, labels, ignorelabels=set(), mtime=0):
+    def savemessagelabels(self, uid, labels, ignorelabels=None, mtime=0):
         """Sets the specified message's labels to the given set.
 
         Note that this function does not check against dryrun settings,
         so you need to ensure that it is never called in a
         dryrun mode."""
 
+        if ignorelabels is None:
+            ignorelabels = set()
+
         raise NotImplementedError
 
     def addmessagelabels(self, uid, labels):
diff --git a/offlineimap/folder/GmailMaildir.py b/offlineimap/folder/GmailMaildir.py
index 4114dd4..ebefd1b 100644
--- a/offlineimap/folder/GmailMaildir.py
+++ b/offlineimap/folder/GmailMaildir.py
@@ -133,12 +133,15 @@ class GmailMaildirFolder(MaildirFolder):
         self.messagelist[uid]['labels'] = labels
         return ret
 
-    def savemessagelabels(self, uid, labels, ignorelabels=set()):
+    def savemessagelabels(self, uid, labels, ignorelabels=None):
         """Change a message's labels to `labels`.
 
         Note that this function does not check against dryrun settings,
         so you need to ensure that it is never called in a dryrun mode."""
 
+        if ignorelabels is None:
+            ignorelabels = set()
+
         filename = self.messagelist[uid]['filename']
         filepath = os.path.join(self.getfullname(), filename)
 
diff --git a/offlineimap/folder/LocalStatus.py b/offlineimap/folder/LocalStatus.py
index d1e2eb8..4f8d42d 100644
--- a/offlineimap/folder/LocalStatus.py
+++ b/offlineimap/folder/LocalStatus.py
@@ -190,13 +190,16 @@ class LocalStatusFolder(BaseFolder):
                 os.close(fd)
 
     # Interface from BaseFolder
-    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=set()):
+    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=None):
         """Writes a new message, with the specified uid.
 
         See folder/Base for detail. Note that savemessage() does not
         check against dryrun settings, so you need to ensure that
         savemessage is never called in a dryrun mode."""
 
+        if labels is None:
+            labels = set()
+
         if uid < 0:
             # We cannot assign a uid.
             return uid
diff --git a/offlineimap/folder/LocalStatusSQLite.py b/offlineimap/folder/LocalStatusSQLite.py
index 5292463..27a9a81 100644
--- a/offlineimap/folder/LocalStatusSQLite.py
+++ b/offlineimap/folder/LocalStatusSQLite.py
@@ -323,13 +323,16 @@ class LocalStatusSQLiteFolder(BaseFolder):
     #        assert False,"getmessageflags() called on non-existing message"
 
     # Interface from BaseFolder
-    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=set()):
+    def savemessage(self, uid, content, flags, rtime, mtime=0, labels=None):
         """Writes a new message, with the specified uid.
 
         See folder/Base for detail. Note that savemessage() does not
         check against dryrun settings, so you need to ensure that
         savemessage is never called in a dryrun mode."""
 
+        if labels is None:
+            labels = set()
+
         if uid < 0:
             # We cannot assign a uid.
             return uid
diff --git a/offlineimap/folder/Maildir.py b/offlineimap/folder/Maildir.py
index f275436..f06217b 100644
--- a/offlineimap/folder/Maildir.py
+++ b/offlineimap/folder/Maildir.py
@@ -270,7 +270,7 @@ class MaildirFolder(BaseFolder):
         filepath = os.path.join(self.getfullname(), filename)
         return os.path.getmtime(filepath)
 
-    def new_message_filename(self, uid, flags=set(), date=None):
+    def new_message_filename(self, uid, flags=None, date=None):
         """Creates a new unique Maildir filename
 
         :param uid: The UID`None`, or a set of maildir flags
@@ -278,6 +278,9 @@ class MaildirFolder(BaseFolder):
         :param flags: (optional) Date
         :returns: String containing unique message filename"""
 
+        if flags is None:
+            flags = set()
+
         timeval, timeseq = _gettimeseq(date)
         uniq_name = '%d_%d.%d.%s,U=%d,FMD5=%s%s2,%s' % \
                     (timeval, timeseq, os.getpid(), socket.gethostname(),