diff --git a/.circleci/config.yml b/.circleci/config.yml
index a56a65b..1d378ee 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -4,14 +4,33 @@ jobs:
     docker:
       - image: python:3
 
-    environment:
-      SECRET_KEY: DEV
-
     steps:
       - checkout
-      - run: pip install pytest coverage
-      - run: pip install -e .
-      - run: coverage run -m pytest
-      - run: coverage report -m
-      - run: pip install codecov
-      - run: codecov
+      - run:
+          name: Install prerequisites
+          command: |
+            pip install pytest coverage
+            pip install -e .
+
+      - run:
+          name: Create config file
+          command: |
+            mkdir -p instance
+            echo << EOF
+            FLASK_APP='flaskr'
+            FLASK_ENV='development'
+            SECRET_KEY='test'
+            SQLALCHEMY_DATABASE_URI='sqlite:////tmp/flaskr.sqlite.db'
+            EOF > instance/config.py
+
+      - run:
+          name: Run coverage tests
+          command: |
+            coverage run -m pytest
+            coverage report -m
+
+      - run:
+          name: Upload to codecov.io
+          command: |
+            pip install codecov
+            codecov
diff --git a/flaskr/__init__.py b/flaskr/__init__.py
index 3974563..f817675 100644
--- a/flaskr/__init__.py
+++ b/flaskr/__init__.py
@@ -2,16 +2,18 @@ import os
 
 from flask import Flask
 
+from flaskr.exceptions import ConfigError
+
 
 def create_app(test_config=None):
     # create and configure the app
     app = Flask(__name__, instance_relative_config=True)
     app.config.from_mapping(
-        SECRET_KEY=os.environ.get("SECRET_KEY", default=None),
+        SECRET_KEY=os.environ.get('SECRET_KEY', default=None),
         SQLALCHEMY_TRACK_MODIFICATIONS=os.environ.get(
-            "SQLALCHEMY_TRACK_MODIFICATIONS", default=False),
+            'SQLALCHEMY_TRACK_MODIFICATIONS', default=False),
         SQLALCHEMY_DATABASE_URI=os.environ.get(
-            "SQLALCHEMY_DATABASE_URI", default=None)
+            'SQLALCHEMY_DATABASE_URI', default=None)
     )
 
     if test_config is None:
@@ -21,11 +23,10 @@ def create_app(test_config=None):
         # load the test config if passed in
         app.config.from_mapping(test_config)
 
-    # ensure the instance folder exists
-    try:
-        os.makedirs(app.instance_path)
-    except OSError:
-        pass
+    if app.config['SECRET_KEY'] is None:
+        raise ConfigError("SECRET_KEY is not defined")
+    if app.config['SQLALCHEMY_DATABASE_URI'] is None:
+        raise ConfigError("SQLALCHEMY_DATABASE_URI is not defined")
 
     from . import db
     db.init_app(app)
diff --git a/flaskr/exceptions.py b/flaskr/exceptions.py
new file mode 100644
index 0000000..131ddee
--- /dev/null
+++ b/flaskr/exceptions.py
@@ -0,0 +1,7 @@
+class Error(Exception):
+    """Base class for exceptions"""
+    pass
+
+class ConfigError(Error):
+    """Raised when a needed configuration is missing"""
+    pass
diff --git a/tests/test_factory.py b/tests/test_factory.py
index caaa0d2..c176c74 100644
--- a/tests/test_factory.py
+++ b/tests/test_factory.py
@@ -1,6 +1,19 @@
+import pytest
+
 from flaskr import create_app
+from flaskr.exceptions import ConfigError
 
 
-def test_config():
-    assert not create_app().testing
-    assert create_app({'TESTING': True}).testing
+@pytest.mark.parametrize('config', (
+    {},
+    {'SECRET_KEY':'test'}
+))
+def test_bad_config(config):
+    try:
+        create_app(config).testing
+        assert False
+    except ConfigError:
+        assert True
+
+def test_good_config():
+    assert create_app(None)