--- a/tests/test.py	Sun Aug 13 16:14:39 2023 +0200
+++ b/tests/test.py	Mon Aug 14 09:31:27 2023 +0200
@@ -2402,6 +2402,33 @@
             cfg.getvar,
             u("intl.cache"))
 
+    def test_global_filter_wrong_syntax(self):
+        cfg = configmix.load()
+        try:
+            y = getattr(cfg, self.interpolate_meth)(u"{{|{{not-existing}}|None}}")
+        except ValueError:
+            pass
+        else:
+            self.fail("`ValueError' should have been raised")
+
+    def test_global_filter_None(self):
+        cfg = configmix.load()
+        y = getattr(cfg, self.interpolate_meth)(u"{{|{{not-existing}}|None|}}")
+        self.assertTrue(y is None)
+
+    def test_global_filter_empty(self):
+        cfg = configmix.load()
+        y = getattr(cfg, self.interpolate_meth)(u"{{|{{not-existing}}|Empty|}}")
+        self.assertEqual(u(""), y)
+
+    def test_global_filter_upper(self):
+        cfg1 = {
+            u("key-1"): u("value for key-1")
+        }
+        cfg = cfg = configmix.config.Configuration(cfg1)
+        y = getattr(cfg, self.interpolate_meth)(u"{{|pre_{{key-1}}_post|upper|}}")
+        self.assertEqual(u("PRE_VALUE FOR KEY-1_POST"), y)
+
 
 class T09Parser(_TParserMixin, unittest.TestCase):