diff --git a/tests/cli/test_main.py b/tests/cli/test_main.py
index bb2bcdd..0fd7d64 100644
--- a/tests/cli/test_main.py
+++ b/tests/cli/test_main.py
@@ -315,6 +315,11 @@ def test_create_collections(subtest, collections):
             set(x.basename for x in tmpdir.join('bar').listdir()) == \
             set(collections)
 
+        result = runner.invoke(
+            ['sync'] + ['foobar/' + x for x in collections]
+        )
+        assert not result.exception
+
 
 def test_ident_conflict(tmpdir, runner):
     runner.write_with_general(dedent('''
diff --git a/vdirsyncer/storage/base.py b/vdirsyncer/storage/base.py
index d86396a..fbdab2c 100644
--- a/vdirsyncer/storage/base.py
+++ b/vdirsyncer/storage/base.py
@@ -77,6 +77,7 @@ class Storage(with_metaclass(StorageMeta)):
         self.read_only = bool(read_only)
 
         if collection and instance_name:
+            assert isinstance(collection, str)
             instance_name = '{}/{}'.format(instance_name, collection)
         self.instance_name = instance_name
         self.collection = collection