diff --git a/nova/test.py b/nova/test.py
index 70ef0a11fd76..d948eb77c12c 100644
--- a/nova/test.py
+++ b/nova/test.py
@@ -726,11 +726,15 @@ class SubclassSignatureTestCase(testtools.TestCase, metaclass=abc.ABCMeta):
         # getmembers returns all members, including members inherited from
         # the base class. It's redundant for us to test these, but as
         # they'll always pass it's not worth the complexity to filter them out.
-        for (name, method) in inspect.getmembers(cls, inspect.ismethod):
+        for (name, method) in inspect.getmembers(cls, inspect.isfunction):
             # Subclass __init__ methods can usually be legitimately different
             if name == '__init__':
                 continue
 
+            # Skip subclass private functions
+            if name.startswith('_'):
+                continue
+
             while hasattr(method, '__wrapped__'):
                 # This is a wrapped function. The signature we're going to
                 # see here is that of the wrapper, which is almost certainly
diff --git a/nova/tests/unit/test_test.py b/nova/tests/unit/test_test.py
index 1042153b10b0..caf3f0b98874 100644
--- a/nova/tests/unit/test_test.py
+++ b/nova/tests/unit/test_test.py
@@ -18,6 +18,7 @@
 
 import os.path
 import tempfile
+import unittest
 from unittest import mock
 import uuid
 
@@ -415,3 +416,21 @@ class PatchOpenTestCase(test.NoDBTestCase):
         without changing other file operations.
         """
         self._test_patched_open()
+
+
+class SubclassSignatureTestCaseTestCase(test.SubclassSignatureTestCase):
+    class Baseclass():
+        def method(self):
+            pass
+
+    class Subclass(Baseclass):
+        def method(self, different):
+            pass
+
+    def _get_base_class(self):
+        return SubclassSignatureTestCaseTestCase.Baseclass
+
+    @unittest.expectedFailure
+    def test_signatures(self):
+        # Test that the signature check fails when it's supposed to fail.
+        super().test_signatures()
diff --git a/nova/virt/libvirt/volume/lightos.py b/nova/virt/libvirt/volume/lightos.py
index 6a22bf6dc63c..c48890997b7f 100644
--- a/nova/virt/libvirt/volume/lightos.py
+++ b/nova/virt/libvirt/volume/lightos.py
@@ -52,7 +52,7 @@ class LibvirtLightOSVolumeDriver(libvirt_volume.LibvirtVolumeDriver):
         super(LibvirtLightOSVolumeDriver, self).disconnect_volume(
             connection_info, instance, force=force)
 
-    def extend_volume(self, connection_info, instance, requested_size=None):
+    def extend_volume(self, connection_info, instance, requested_size):
         """Extend the volume."""
         LOG.debug("calling os-brick to extend LightOS Volume."
                   "instance:%s, volume_id:%s",