diff --git a/deckhand/engine/secrets_manager.py b/deckhand/engine/secrets_manager.py
index dd44ef2b..1985586b 100644
--- a/deckhand/engine/secrets_manager.py
+++ b/deckhand/engine/secrets_manager.py
@@ -353,7 +353,7 @@ class SecretsSubstitution(object):
                 else:
                     message = ('Could not find substitution source document '
                                '[%s] %s among the provided substitution '
-                               'sources.', src_schema, src_name)
+                               'sources.' % (src_schema, src_name))
                     if self._fail_on_missing_sub_src:
                         LOG.error(message)
                         raise errors.SubstitutionSourceNotFound(
diff --git a/deckhand/tests/unit/engine/test_document_layering.py b/deckhand/tests/unit/engine/test_document_layering.py
index b4bc8971..a4e68600 100644
--- a/deckhand/tests/unit/engine/test_document_layering.py
+++ b/deckhand/tests/unit/engine/test_document_layering.py
@@ -134,7 +134,7 @@ class TestDocumentLayeringScenarios(TestDocumentLayering):
         self._test_layering(documents, site_expected={},
                             fail_on_missing_sub_src=False)
         self.assertTrue(m_log.warning.called)
-        self.assertRegex(m_log.warning.mock_calls[0][1][0][0],
+        self.assertRegex(m_log.warning.mock_calls[0][1][0],
                          r'Could not find substitution source document .*')
 
     def test_layering_substitution_source_skips_layering(self):