diff --git a/openstack/common/log.py b/openstack/common/log.py
index 3b1b557f..44f48902 100644
--- a/openstack/common/log.py
+++ b/openstack/common/log.py
@@ -410,18 +410,20 @@ def setup(product_name, version='unknown'):
     sys.excepthook = _create_logging_excepthook(product_name)
 
 
-def set_defaults(logging_context_format_string,
+def set_defaults(logging_context_format_string=None,
                  default_log_levels=None):
     # Just in case the caller is not setting the
     # default_log_level. This is insurance because
     # we introduced the default_log_level parameter
     # later in a backwards in-compatible change
-    if default_log_levels is None:
-        default_log_levels = DEFAULT_LOG_LEVELS
-    cfg.set_defaults(
-            log_opts,
-            logging_context_format_string=logging_context_format_string,
-            default_log_levels=default_log_levels)
+    if default_log_levels is not None:
+        cfg.set_defaults(
+                log_opts,
+                default_log_levels=default_log_levels)
+    if logging_context_format_string is not None:
+        cfg.set_defaults(
+                log_opts,
+                logging_context_format_string=logging_context_format_string)
 
 
 def _find_facility_from_conf():
diff --git a/tests/unit/test_log.py b/tests/unit/test_log.py
index 6439a534..a50578f5 100644
--- a/tests/unit/test_log.py
+++ b/tests/unit/test_log.py
@@ -541,11 +541,6 @@ class SetDefaultsTestCase(test_base.BaseTestCase):
         for opt in log.log_opts:
             opt.default = self._orig_defaults[opt.dest]
 
-    def test_default_to_none(self):
-        log.set_defaults(logging_context_format_string=None)
-        self.conf([])
-        self.assertIsNone(self.conf.logging_context_format_string)
-
     def test_default_log_level_to_none(self):
         log.set_defaults(logging_context_format_string=None,
                          default_log_levels=None)
@@ -561,10 +556,10 @@ class SetDefaultsTestCase(test_base.BaseTestCase):
         self.assertEqual(self.conf.logging_context_format_string, my_default)
 
     def test_change_default_log_level(self):
-        log.set_defaults(logging_context_format_string=None,
-                         default_log_levels=['foo=bar'])
+        log.set_defaults(default_log_levels=['foo=bar'])
         self.conf([])
         self.assertEqual(['foo=bar'], self.conf.default_log_levels)
+        self.assertIsNotNone(self.conf.logging_context_format_string)
 
 
 class LogConfigOptsTestCase(test_base.BaseTestCase):