Chaining boltons.urlutils.URL.navigate() results in TypeError

boltons/urlutils.py

@@ -654,7 +654,8 @@ def navigate(self, dest):
             if dest.path.startswith(u'/'):   # absolute path
                 new_path_parts = list(dest.path_parts)
             else:  # relative path
-                new_path_parts = self.path_parts[:-1] + dest.path_parts
+                new_path_parts = list(self.path_parts[:-1]) \
+                               + list(dest.path_parts)
         else:
             new_path_parts = list(self.path_parts)
             if not query_params:

tests/test_urlutils.py

@@ -313,6 +313,25 @@ def test_navigate():
     assert navd.to_text() == _dest_text
 
 
+@pytest.mark.parametrize(
+    ('expected', 'base', 'paths'), [
+    ('https://host/b', 'https://host', ('a', '/b', )),
+    ('https://host/b', 'https://host', ('a', 'b', )),
+    ('https://host/a/b', 'https://host', ('a/', 'b', )),
+    ('https://host/b', 'https://host', ('/a', 'b', )),
+    ('https://host/a/b', 'https://host/a/', (None, 'b', )),
+    ('https://host/b', 'https://host/a', (None, 'b', )),
+])
+def test_chained_navigate(expected, base, paths):
+    """Chained :meth:`navigate` calls produces correct results."""
+    url = URL(base)
+
+    for path in paths:
+        url = url.navigate(path)
+
+    assert expected == url.to_text()
+
+
 # TODO: RFC3986 6.2.3 (not just for query add, either)
 # def test_add_query():
 #     url = URL('http://www.example.com')