--- a/tests/test.py	Fri Dec 31 21:24:16 2021 +0100
+++ b/tests/test.py	Sat Jan 01 18:01:32 2022 +0100
@@ -1777,7 +1777,7 @@
         e = configmix.quote(u"")
         self.assertEqual(u"", e)
         self.assertEqual(u"", self.unquote(e))
-        
+
     def test_quoting_and_unquoting_are_inverse(self):
         for c in u"""%.:#|"'{}[]""":
             qc = configmix.quote(c)
@@ -1874,12 +1874,36 @@
         p = self.pathstr2path(u"a%x2Eb.c%u002Ed.e%U0000002Ef")
         self.assertEqual((u"a.b", u"c.d", u"e.f"), p)
 
+    def test_split_ns_empty(self):
+        self.assertEqual((None, u""), self.split_ns(u""))
+
+    def test_split_ns_empty_parts(self):
+        self.assertEqual((u"", u""), self.split_ns(u":"))
+
+    def test_split_ns_no_ns(self):
+        self.assertEqual((None, u"the-varname"), self.split_ns(u"the-varname"))
+
+    def test_split_ns_non_quoted(self):
+        self.assertEqual(
+            (u"the-ns", "the-rest:with:colons|filter1|filter2"),
+            self.split_ns(u"the-ns:the-rest:with:colons|filter1|filter2"))
+
+    def test_split_ns_quoting(self):
+        self.assertEqual((u":", u"%x3a"), self.split_ns(u"%x3a:%x3a"))
+
 
 class T09Parser(_TParserMixin, unittest.TestCase):
 
     def setUp(self):
         self.unquote = configmix.config.py_unquote
         self.pathstr2path = configmix.config.py_pathstr2path
+        self.split_ns = configmix.config._py_split_ns
+
+    def test_split_ns_wrong_type(self):
+        self.assertRaises(
+            AttributeError,    # no .partition
+            self.split_ns,
+            1)
 
 
 if configmix.config.fast_unquote is not None:
@@ -1888,6 +1912,13 @@
         def setUp(self):
             self.unquote = configmix.config.fast_unquote
             self.pathstr2path = configmix.config.fast_pathstr2path
+            self.split_ns = configmix.config._fast_split_ns
+
+        def test_split_ns_wrong_type(self):
+            self.assertRaises(
+                TypeError,
+                self.split_ns,
+                b":")
 
 
 if __name__ == "__main__":