diff --git a/ironic/common/exception.py b/ironic/common/exception.py
index e5630990c5..4cdbb03dd4 100644
--- a/ironic/common/exception.py
+++ b/ironic/common/exception.py
@@ -128,10 +128,12 @@ class IronicException(Exception):
 
     def __str__(self):
         """Encode to utf-8 then wsme api can consume it as well."""
-        if not six.PY3:
-            return six.text_type(self.args[0]).encode('utf-8')
-
-        return self.args[0]
+        value = self.__unicode__()
+        if six.PY3:
+            # On Python 3 unicode is the same as str
+            return value
+        else:
+            return value.encode('utf-8')
 
     def __unicode__(self):
         """Return a unicode representation of the exception message."""
diff --git a/ironic/drivers/modules/deploy_utils.py b/ironic/drivers/modules/deploy_utils.py
index f8b81e9f77..30c8ae6528 100644
--- a/ironic/drivers/modules/deploy_utils.py
+++ b/ironic/drivers/modules/deploy_utils.py
@@ -829,7 +829,7 @@ def validate_image_properties(ctx, deploy_info, properties):
         raise exception.InvalidParameterValue(_(
             "Image %s can not be found.") % image_href)
     except exception.ImageRefValidationFailed as e:
-        raise exception.InvalidParameterValue(e)
+        raise exception.InvalidParameterValue(err=e)
 
     missing_props = []
     for prop in properties:
diff --git a/ironic/tests/unit/common/test_exception.py b/ironic/tests/unit/common/test_exception.py
index 17a80b9fed..5e7f6bcd60 100644
--- a/ironic/tests/unit/common/test_exception.py
+++ b/ironic/tests/unit/common/test_exception.py
@@ -32,7 +32,7 @@ class TestException(exception.IronicException):
 
 
 class TestIronicException(base.TestCase):
-    def test___init__(self):
+    def test___str__encoding(self):
         expected = b'\xc3\xa9\xe0\xaf\xb2\xe0\xbe\x84'
         if six.PY3:
             expected = expected.decode('utf-8')
@@ -40,6 +40,11 @@ class TestIronicException(base.TestCase):
         exc = exception.IronicException(message)
         self.assertEqual(expected, exc.__str__())
 
+    def test___str__non_string(self):
+        exc = exception.IronicException(42)
+        self.assertEqual("42", exc.__str__())
+        self.assertEqual(u"42", exc.__unicode__())
+
     @mock.patch.object(exception.LOG, 'error', autospec=True)
     def test___init___invalid_kwarg(self, log_mock):
         self.config(fatal_exception_format_errors=False)
diff --git a/ironic/tests/unit/drivers/modules/test_deploy_utils.py b/ironic/tests/unit/drivers/modules/test_deploy_utils.py
index 4f6d828103..779605de6a 100644
--- a/ironic/tests/unit/drivers/modules/test_deploy_utils.py
+++ b/ironic/tests/unit/drivers/modules/test_deploy_utils.py
@@ -1955,9 +1955,10 @@ class ValidateImagePropertiesTestCase(db_base.DbTestCase):
             driver_internal_info=DRV_INTERNAL_INFO_DICT,
         )
         inst_info = utils.get_image_instance_info(node)
-        self.assertRaises(exception.InvalidParameterValue,
-                          utils.validate_image_properties, self.context,
-                          inst_info, ['kernel', 'ramdisk'])
+        self.assertRaisesRegex(exception.InvalidParameterValue,
+                               'HTTPError',
+                               utils.validate_image_properties, self.context,
+                               inst_info, ['kernel', 'ramdisk'])
 
 
 class ValidateParametersTestCase(db_base.DbTestCase):
diff --git a/releasenotes/notes/type-error-str-6826c53d7e5e1243.yaml b/releasenotes/notes/type-error-str-6826c53d7e5e1243.yaml
new file mode 100644
index 0000000000..e144f1c1a1
--- /dev/null
+++ b/releasenotes/notes/type-error-str-6826c53d7e5e1243.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Returns the correct error message on providing an invalid reference to
+    ``image_source``. Previously an internal error was raised.