diff --git a/ironic/api/controllers/v1/types.py b/ironic/api/controllers/v1/types.py
index fdd45fb4da..ffcaa7b7f1 100644
--- a/ironic/api/controllers/v1/types.py
+++ b/ironic/api/controllers/v1/types.py
@@ -196,12 +196,12 @@ class ListType(wtypes.UserType):
         """Validate and convert the input to a ListType.
 
         :param value: A comma separated string of values
-        :returns: A list of values.
+        :returns: A list of unique values, whose order is not guaranteed.
         """
         items = [v.strip().lower() for v in six.text_type(value).split(',')]
         # filter() to remove empty items
         # set() to remove duplicated items
-        return set(filter(None, items))
+        return list(set(filter(None, items)))
 
     @staticmethod
     def frombasetype(value):
diff --git a/ironic/tests/unit/api/v1/test_types.py b/ironic/tests/unit/api/v1/test_types.py
index 9fb7747c57..ac7ce73e95 100644
--- a/ironic/tests/unit/api/v1/test_types.py
+++ b/ironic/tests/unit/api/v1/test_types.py
@@ -277,3 +277,4 @@ class TestListType(base.TestCase):
                               v.validate("foo, ,,bar"))
         self.assertItemsEqual(['foo', 'bar'],
                               v.validate("foo,foo,foo,bar"))
+        self.assertIsInstance(v.validate('foo,bar'), list)