diff --git a/ironic/conf/pxe.py b/ironic/conf/pxe.py
index 042046001d..f16aa94e04 100644
--- a/ironic/conf/pxe.py
+++ b/ironic/conf/pxe.py
@@ -36,7 +36,7 @@ opts = [
                default='/var/lib/ironic/master_images',
                help=_('On the ironic-conductor node, directory where master '
                       'instance images are stored on disk. '
-                      'Setting to <None> disables image caching.')),
+                      'Setting to the empty string disables image caching.')),
     cfg.IntOpt('image_cache_size',
                default=20480,
                help=_('Maximum size (in MiB) of cache for master images, '
diff --git a/ironic/drivers/modules/deploy_utils.py b/ironic/drivers/modules/deploy_utils.py
index 25c4d381d9..c2d47edd9c 100644
--- a/ironic/drivers/modules/deploy_utils.py
+++ b/ironic/drivers/modules/deploy_utils.py
@@ -1113,8 +1113,9 @@ def direct_deploy_should_convert_raw_image(node):
 class InstanceImageCache(image_cache.ImageCache):
 
     def __init__(self):
+        master_path = CONF.pxe.instance_master_path or None
         super(self.__class__, self).__init__(
-            CONF.pxe.instance_master_path,
+            master_path,
             # MiB -> B
             cache_size=CONF.pxe.image_cache_size * 1024 * 1024,
             # min -> sec
diff --git a/ironic/tests/unit/drivers/modules/test_deploy_utils.py b/ironic/tests/unit/drivers/modules/test_deploy_utils.py
index a4086ee4fa..4a16a5d9d6 100644
--- a/ironic/tests/unit/drivers/modules/test_deploy_utils.py
+++ b/ironic/tests/unit/drivers/modules/test_deploy_utils.py
@@ -2761,3 +2761,29 @@ class TestStorageInterfaceUtils(db_base.DbTestCase):
         with task_manager.acquire(
                 self.context, self.node.uuid, shared=False) as task:
             self.assertFalse(utils.is_iscsi_boot(task))
+
+
+class InstanceImageCacheTestCase(db_base.DbTestCase):
+    @mock.patch.object(fileutils, 'ensure_tree')
+    def test_with_master_path(self, mock_ensure_tree):
+        self.config(instance_master_path='/fake/path', group='pxe')
+        self.config(image_cache_size=500, group='pxe')
+        self.config(image_cache_ttl=30, group='pxe')
+
+        cache = utils.InstanceImageCache()
+
+        mock_ensure_tree.assert_called_once_with('/fake/path')
+        self.assertEqual(500 * 1024 * 1024, cache._cache_size)
+        self.assertEqual(30 * 60, cache._cache_ttl)
+
+    @mock.patch.object(fileutils, 'ensure_tree')
+    def test_without_master_path(self, mock_ensure_tree):
+        self.config(instance_master_path='', group='pxe')
+        self.config(image_cache_size=500, group='pxe')
+        self.config(image_cache_ttl=30, group='pxe')
+
+        cache = utils.InstanceImageCache()
+
+        mock_ensure_tree.assert_not_called()
+        self.assertEqual(500 * 1024 * 1024, cache._cache_size)
+        self.assertEqual(30 * 60, cache._cache_ttl)
diff --git a/releasenotes/notes/fix-instance-master-path-config-fa524c907a7888e5.yaml b/releasenotes/notes/fix-instance-master-path-config-fa524c907a7888e5.yaml
new file mode 100644
index 0000000000..4184a4cd76
--- /dev/null
+++ b/releasenotes/notes/fix-instance-master-path-config-fa524c907a7888e5.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixes an issue where the master instance image cache could not be disabled.
+    The configuration option ``[pxe]/instance_master_path`` may now be set to
+    the empty string to disable the cache.