diff --git a/mistral/exceptions.py b/mistral/exceptions.py
index 3fdddd4e4..8d151153e 100644
--- a/mistral/exceptions.py
+++ b/mistral/exceptions.py
@@ -29,7 +29,8 @@ class MistralException(ex.Error):
     def __str__(self):
         return self.message
 
-    def __init__(self, message):
+    def __init__(self, message=message):
+        self.message = message
         super(MistralException, self).__init__(
             '%s: %s' % (self.code, self.message))
 
diff --git a/mistral/tests/api/base.py b/mistral/tests/api/base.py
index 6a1da3702..2660e5035 100644
--- a/mistral/tests/api/base.py
+++ b/mistral/tests/api/base.py
@@ -16,6 +16,7 @@
 
 import pecan
 import pecan.testing
+from webtest.app import AppError
 
 from oslo.config import cfg
 
@@ -55,3 +56,12 @@ class FunctionalTest(test_base.DbTestCase):
     def tearDown(self):
         super(FunctionalTest, self).tearDown()
         pecan.set_config({}, overwrite=True)
+
+    def assertNotFound(self, url):
+        try:
+            self.app.get(url, headers={'Accept': 'application/json'})
+        except AppError as error:
+            if hasattr(error, 'message'):
+                self.assertIn('Bad response: 404 Not Found', error.message)
+                return
+        self.fail('Expected 404 Not found but got OK')
diff --git a/mistral/tests/api/v1/controllers/test_executions.py b/mistral/tests/api/v1/controllers/test_executions.py
index 7336a994d..9d5e7200f 100644
--- a/mistral/tests/api/v1/controllers/test_executions.py
+++ b/mistral/tests/api/v1/controllers/test_executions.py
@@ -16,6 +16,8 @@
 
 import mock
 
+from mistral import exceptions as ex
+from webtest.app import AppError
 from mistral.tests.api import base
 from mistral.db import api as db_api
 from mistral.engine import engine
@@ -52,6 +54,11 @@ class TestExecutionsController(base.FunctionalTest):
         self.assertEqual(resp.status_int, 200)
         self.assertDictEqual(EXECS[0], resp.json)
 
+    @mock.patch.object(db_api, "execution_get",
+                       mock.MagicMock(return_value=None))
+    def test_get_empty(self):
+        self.assertNotFound('/v1/workbooks/my_workbook/executions/123')
+
     @mock.patch.object(db_api, "execution_update",
                        mock.MagicMock(return_value=UPDATED_EXEC))
     def test_put(self):
@@ -66,10 +73,17 @@ class TestExecutionsController(base.FunctionalTest):
     def test_post(self):
         resp = self.app.post_json('/v1/workbooks/my_workbook/executions',
                                   EXECS[0])
-
         self.assertEqual(resp.status_int, 201)
         self.assertDictEqual(EXECS[0], resp.json)
 
+    @mock.patch.object(engine, "start_workflow_execution",
+                       mock.MagicMock(side_effect=ex.MistralException))
+    def test_post_throws_exception(self):
+        with self.assertRaises(AppError) as context:
+            self.app.post_json('/v1/workbooks/my_workbook/executions',
+                               EXECS[0])
+        self.assertIn('Bad response: 400', context.exception.message)
+
     @mock.patch.object(db_api, "execution_delete",
                        mock.MagicMock(return_value=None))
     def test_delete(self):