diff --git a/tests/storage/__init__.py b/tests/storage/__init__.py
index b72655a..63450b7 100644
--- a/tests/storage/__init__.py
+++ b/tests/storage/__init__.py
@@ -14,7 +14,7 @@ from .. import EVENT_TEMPLATE, TASK_TEMPLATE, VCARD_TEMPLATE, \
 
 def format_item(item_template, uid=None):
     # assert that special chars are handled correctly.
-    r = '{}@vdirsyncer'.format(random.random())
+    r = random.random()
     return Item(item_template.format(r=r, uid=uid or r))
 
 
diff --git a/tests/storage/dav/servers/radicale/__init__.py b/tests/storage/dav/servers/radicale/__init__.py
index d25a189..1b7bd44 100644
--- a/tests/storage/dav/servers/radicale/__init__.py
+++ b/tests/storage/dav/servers/radicale/__init__.py
@@ -113,7 +113,7 @@ class ServerMixin(object):
                 url = url.rstrip('/') + '/' + collection
 
             rv = {'url': url, 'username': 'bob', 'password': 'bob',
-                  'collection': collection, 'unsafe_href_chars': ''}
+                  'collection': collection}
 
             if collection is not None:
                 s = self.storage_class(**rv)
diff --git a/tests/storage/test_filesystem.py b/tests/storage/test_filesystem.py
index d4f6d54..11b5cc2 100644
--- a/tests/storage/test_filesystem.py
+++ b/tests/storage/test_filesystem.py
@@ -44,7 +44,7 @@ class TestFilesystemStorage(StorageTests):
         s = self.storage_class(str(tmpdir), '.txt')
         s.upload(Item(u'UID:a/b/c'))
         item_file, = tmpdir.listdir()
-        assert str(item_file).endswith('a_b_c.txt')
+        assert '/' not in item_file.basename and item_file.isfile()
 
     def test_too_long_uid(self, tmpdir):
         s = self.storage_class(str(tmpdir), '.txt')
diff --git a/vdirsyncer/storage/dav.py b/vdirsyncer/storage/dav.py
index 2e693af..eb0ce46 100644
--- a/vdirsyncer/storage/dav.py
+++ b/vdirsyncer/storage/dav.py
@@ -341,15 +341,14 @@ class DavStorage(Storage):
     }
 
     def __init__(self, url, username='', password='', verify=True, auth=None,
-                 useragent=USERAGENT, unsafe_href_chars='@',
-                 verify_fingerprint=None, auth_cert=None, **kwargs):
+                 useragent=USERAGENT, verify_fingerprint=None, auth_cert=None,
+                 **kwargs):
         super(DavStorage, self).__init__(**kwargs)
 
         url = url.rstrip('/') + '/'
         self.session = DavSession(url, username, password, verify, auth,
                                   useragent, verify_fingerprint,
                                   auth_cert)
-        self.unsafe_href_chars = unsafe_href_chars
 
         # defined for _repr_attributes
         self.username = username
@@ -369,7 +368,7 @@ class DavStorage(Storage):
         return _normalize_href(self.session.url, *args, **kwargs)
 
     def _get_href(self, item):
-        href = utils.generate_href(item.ident, unsafe=self.unsafe_href_chars)
+        href = utils.generate_href(item.ident)
         return self._normalize_href(href + self.fileext)
 
     def _is_item_mimetype(self, mimetype):
diff --git a/vdirsyncer/storage/filesystem.py b/vdirsyncer/storage/filesystem.py
index 7b844ae..19b985a 100644
--- a/vdirsyncer/storage/filesystem.py
+++ b/vdirsyncer/storage/filesystem.py
@@ -87,9 +87,7 @@ class FilesystemStorage(Storage):
         return os.path.join(self.path, to_native(href, self.encoding))
 
     def _get_href(self, ident):
-        # XXX: POSIX only defines / and \0 as invalid chars, but we should make
-        # this work crossplatform.
-        return generate_href(ident, '/') + self.fileext
+        return generate_href(ident) + self.fileext
 
     def list(self):
         for fname in os.listdir(self.path):
diff --git a/vdirsyncer/utils/__init__.py b/vdirsyncer/utils/__init__.py
index 8f88311..5f926a4 100644
--- a/vdirsyncer/utils/__init__.py
+++ b/vdirsyncer/utils/__init__.py
@@ -8,6 +8,11 @@ from .compat import iteritems, to_unicode
 from .. import exceptions
 
 
+SAFE_UID_CHARS = ('abcdefghijklmnopqrstuvwxyz'
+                  'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+                  '0123456789_.-+')
+
+
 _missing = object()
 
 
@@ -159,10 +164,8 @@ class cached_property(object):
         return result
 
 
-def generate_href(ident=None, unsafe=''):
-    if ident and \
-       ident.encode('ascii', 'ignore').decode('ascii') == ident:
-        for char in unsafe:
-            ident = ident.replace(char, '_')
+def generate_href(ident=None, safe=SAFE_UID_CHARS):
+    if not ident or set(ident) - set(safe):
+        return to_unicode(uuid.uuid4().hex)
+    else:
         return ident
-    return to_unicode(uuid.uuid4().hex)