diff --git a/cinder/tests/unit/volume/drivers/test_rbd.py b/cinder/tests/unit/volume/drivers/test_rbd.py
index d7fbd7c0855..62f603a50ff 100644
--- a/cinder/tests/unit/volume/drivers/test_rbd.py
+++ b/cinder/tests/unit/volume/drivers/test_rbd.py
@@ -1544,9 +1544,9 @@ class RBDTestCase(test.TestCase):
                                return_value=dynamic_total):
             result = self.driver._get_pool_stats()
         client.cluster.mon_command.assert_has_calls([
-            mock.call('{"prefix":"df", "format":"json"}', ''),
+            mock.call('{"prefix":"df", "format":"json"}', b''),
             mock.call('{"prefix":"osd pool get-quota", "pool": "rbd",'
-                      ' "format":"json"}', ''),
+                      ' "format":"json"}', b''),
         ])
         self.assertEqual((free_capacity, total_capacity), result)
 
@@ -1567,9 +1567,9 @@ class RBDTestCase(test.TestCase):
         ]
         result = self.driver._get_pool_stats()
         client.cluster.mon_command.assert_has_calls([
-            mock.call('{"prefix":"df", "format":"json"}', ''),
+            mock.call('{"prefix":"df", "format":"json"}', b''),
             mock.call('{"prefix":"osd pool get-quota", "pool": "rbd",'
-                      ' "format":"json"}', ''),
+                      ' "format":"json"}', b''),
         ])
         free_capacity = 1.56
         total_capacity = 3.0
diff --git a/cinder/volume/drivers/rbd.py b/cinder/volume/drivers/rbd.py
index 068821c1caf..f14ee13c192 100644
--- a/cinder/volume/drivers/rbd.py
+++ b/cinder/volume/drivers/rbd.py
@@ -558,14 +558,14 @@ class RBDDriver(driver.CloneableImageVD, driver.MigrateVD,
 
         with RADOSClient(self) as client:
             ret, df_outbuf, __ = client.cluster.mon_command(
-                '{"prefix":"df", "format":"json"}', '')
+                '{"prefix":"df", "format":"json"}', b'')
             if ret:
                 LOG.warning('Unable to get rados pool stats.')
                 return 'unknown', 'unknown'
 
             ret, quota_outbuf, __ = client.cluster.mon_command(
                 '{"prefix":"osd pool get-quota", "pool": "%s",'
-                ' "format":"json"}' % pool_name, '')
+                ' "format":"json"}' % pool_name, b'')
             if ret:
                 LOG.warning('Unable to get rados pool quotas.')
                 return 'unknown', 'unknown'
diff --git a/releasenotes/notes/bug-1913449-4796b366ae7e871b.yaml b/releasenotes/notes/bug-1913449-4796b366ae7e871b.yaml
new file mode 100644
index 00000000000..4395292b43d
--- /dev/null
+++ b/releasenotes/notes/bug-1913449-4796b366ae7e871b.yaml
@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    `Bug 1913449 <https://bugs.launchpad.net/cinder/+bug/1913449>`_:
+    Fix RBD driver _update_volume_stats() failing when using Ceph
+    Pacific python rados libraries.  This failed because we
+    were passing a str instead of bytes to cluster.mon_command()