diff --git a/tests/storage/__init__.py b/tests/storage/__init__.py
index d9b2ca5..e859914 100644
--- a/tests/storage/__init__.py
+++ b/tests/storage/__init__.py
@@ -71,23 +71,23 @@ class StorageTests(object):
         s = self._get_storage()
         item = self._create_bogus_item(1)
         with pytest.raises(exceptions.PreconditionFailed):
-            s.update(s._get_href('1'), item, 123)
+            s.update(s._get_href('1'), item, '"123"')
         with pytest.raises(exceptions.PreconditionFailed):
-            s.update('huehue', item, 123)
+            s.update('huehue', item, '"123"')
 
     def test_wrong_etag(self):
         s = self._get_storage()
         obj = self._create_bogus_item(1)
         href, etag = s.upload(obj)
         with pytest.raises(exceptions.PreconditionFailed):
-            s.update(href, obj, 'lolnope')
+            s.update(href, obj, '"lolnope"')
         with pytest.raises(exceptions.PreconditionFailed):
-            s.delete(href, 'lolnope')
+            s.delete(href, '"lolnope"')
 
     def test_delete_nonexisting(self):
         s = self._get_storage()
         with pytest.raises(exceptions.PreconditionFailed):
-            s.delete('1', 123)
+            s.delete('1', '"123"')
 
     def test_list(self):
         s = self._get_storage()
diff --git a/vdirsyncer/storage/dav/base.py b/vdirsyncer/storage/dav/base.py
index 5e86be0..9e57aca 100644
--- a/vdirsyncer/storage/dav/base.py
+++ b/vdirsyncer/storage/dav/base.py
@@ -193,6 +193,7 @@ class DavStorage(Storage):
         if etag is None:
             headers['If-None-Match'] = '*'
         else:
+            assert etag[0] == etag[-1] == '"'
             headers['If-Match'] = etag
 
         response = self._request(
@@ -221,6 +222,7 @@ class DavStorage(Storage):
     def delete(self, href, etag):
         href = self._normalize_href(href)
         headers = self._default_headers()
+        assert etag[0] == etag[-1] == '"'
         headers.update({
             'If-Match': etag
         })