diff --git a/docs/vdir.rst b/docs/vdir.rst
index 0d510c8..3c5a1e0 100644
--- a/docs/vdir.rst
+++ b/docs/vdir.rst
@@ -20,7 +20,7 @@ An item is:
 An item *should* contain a ``UID`` property as described by the vCard and
 iCalendar standards.
 
-The filename *must* consist of the ``ident``, followed by the file extension.
+The filename *should* consist of the ``ident``, followed by the file extension.
 The ``ident`` is either the ``UID``, if the item has one, else a string with
 similar properties as the ``UID``:
 
@@ -32,6 +32,9 @@ similar properties as the ``UID``:
 
    -- The vCard_ RFC
 
+Character limitations imposed by the filesystem *should* be circumvented by
+replacing the offending characters with underscores ``_``.
+
 One reason this format was chosen is due to its compatibility with the CardDAV_
 and CalDAV_ standards.
 
diff --git a/tests/storage/test_filesystem.py b/tests/storage/test_filesystem.py
index 8921522..5097307 100644
--- a/tests/storage/test_filesystem.py
+++ b/tests/storage/test_filesystem.py
@@ -12,6 +12,7 @@ import os
 
 import pytest
 
+from vdirsyncer.utils.vobject import Item
 from vdirsyncer.storage.filesystem import FilesystemStorage
 
 from . import StorageTests
@@ -55,3 +56,9 @@ class TestFilesystemStorage(StorageTests):
         with pytest.raises(TypeError):
             s.upload(BrokenItem)
         assert not tmpdir.listdir()
+
+    def test_ident_with_slash(self, tmpdir):
+        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')
diff --git a/vdirsyncer/storage/filesystem.py b/vdirsyncer/storage/filesystem.py
index e47b7bc..19be94e 100644
--- a/vdirsyncer/storage/filesystem.py
+++ b/vdirsyncer/storage/filesystem.py
@@ -62,7 +62,9 @@ class FilesystemStorage(Storage):
         return os.path.join(self.path, href)
 
     def _get_href(self, item):
-        return item.ident + self.fileext
+        # XXX: POSIX only defines / and \0 as invalid chars, but we should make
+        # this work crossplatform.
+        return item.ident.replace('/', '_') + self.fileext
 
     def list(self):
         for fname in os.listdir(self.path):