diff --git a/test/unit/test_providers_kms_master_key_provider.py b/test/unit/test_providers_kms_master_key_provider.py
index 48802a36f..9a122e622 100644
--- a/test/unit/test_providers_kms_master_key_provider.py
+++ b/test/unit/test_providers_kms_master_key_provider.py
@@ -12,6 +12,7 @@
 # language governing permissions and limitations under the License.
 """Unit test suite from aws_encryption_sdk.key_providers.kms.KMSMasterKeyProvider"""
 import botocore.client
+import botocore.session
 import pytest
 from mock import ANY, MagicMock, call, patch, sentinel
 
@@ -22,6 +23,19 @@
 pytestmark = [pytest.mark.unit, pytest.mark.local]
 
 
+@pytest.fixture(autouse=True, params=[True, False], ids=["default region", "no default region"])
+def patch_default_region(request, monkeypatch):
+    """Run all tests in this module both with a default region set and no default region set.
+
+    This ensures that we do not regress on default region handling.
+    https://github.com/aws/aws-encryption-sdk-python/issues/31
+    """
+    if request.param:
+        monkeypatch.setenv("AWS_DEFAULT_REGION", "us-west-2")
+    else:
+        monkeypatch.delenv("AWS_DEFAULT_REGION", raising=False)
+
+
 def test_init_with_regionless_key_ids_and_region_names():
     key_ids = ("alias/key_1",)
     region_names = ("test-region-1",)
@@ -32,6 +46,7 @@ def test_init_with_regionless_key_ids_and_region_names():
 class TestKMSMasterKeyProvider(object):
     @pytest.fixture(autouse=True)
     def apply_fixtures(self):
+        self.botocore_no_region_session = botocore.session.Session(session_vars={"region": (None, None, None, None)})
         self.mock_botocore_session_patcher = patch("aws_encryption_sdk.key_providers.kms.botocore.session.Session")
         self.mock_botocore_session = self.mock_botocore_session_patcher.start()
         self.mock_boto3_session_patcher = patch("aws_encryption_sdk.key_providers.kms.boto3.session.Session")
@@ -69,7 +84,7 @@ def test_init_with_region_names(self, mock_add_clients):
 
     @patch("aws_encryption_sdk.key_providers.kms.KMSMasterKeyProvider.add_regional_client")
     def test_init_with_default_region_found(self, mock_add_regional_client):
-        test = KMSMasterKeyProvider()
+        test = KMSMasterKeyProvider(botocore_session=self.botocore_no_region_session)
         assert test.default_region is None
         with patch.object(
             test.config.botocore_session, "get_config_variable", return_value=sentinel.default_region
@@ -77,11 +92,11 @@ def test_init_with_default_region_found(self, mock_add_regional_client):
             test._process_config()
             mock_get_config.assert_called_once_with("region")
             assert test.default_region is sentinel.default_region
-            mock_add_regional_client.assert_called_once_with(sentinel.default_region)
+            mock_add_regional_client.assert_called_with(sentinel.default_region)
 
     @patch("aws_encryption_sdk.key_providers.kms.KMSMasterKeyProvider.add_regional_client")
     def test_init_with_default_region_not_found(self, mock_add_regional_client):
-        test = KMSMasterKeyProvider()
+        test = KMSMasterKeyProvider(botocore_session=self.botocore_no_region_session)
         assert test.default_region is None
         with patch.object(test.config.botocore_session, "get_config_variable", return_value=None) as mock_get_config:
             test._process_config()
@@ -93,12 +108,12 @@ def test_add_regional_client_new(self):
         test = KMSMasterKeyProvider()
         test._regional_clients = {}
         test.add_regional_client("ex_region_name")
-        self.mock_boto3_session.assert_called_once_with(region_name="ex_region_name", botocore_session=ANY)
-        self.mock_boto3_session_instance.client.assert_called_once_with("kms", config=test._user_agent_adding_config)
+        self.mock_boto3_session.assert_called_with(region_name="ex_region_name", botocore_session=ANY)
+        self.mock_boto3_session_instance.client.assert_called_with("kms", config=test._user_agent_adding_config)
         assert test._regional_clients["ex_region_name"] is self.mock_boto3_client_instance
 
     def test_add_regional_client_exists(self):
-        test = KMSMasterKeyProvider()
+        test = KMSMasterKeyProvider(botocore_session=self.botocore_no_region_session)
         test._regional_clients["ex_region_name"] = sentinel.existing_client
         test.add_regional_client("ex_region_name")
         assert not self.mock_boto3_session.called
@@ -114,7 +129,7 @@ def test_client_valid_region_name(self, mock_add_client):
         test = KMSMasterKeyProvider()
         test._regional_clients["us-east-1"] = self.mock_boto3_client_instance
         client = test._client("arn:aws:kms:us-east-1:222222222222:key/aaaaaaaa-1111-2222-3333-bbbbbbbbbbbb")
-        mock_add_client.assert_called_once_with("us-east-1")
+        mock_add_client.assert_called_with("us-east-1")
         assert client is self.mock_boto3_client_instance
 
     @patch("aws_encryption_sdk.key_providers.kms.KMSMasterKeyProvider.add_regional_client")
@@ -124,10 +139,10 @@ def test_client_no_region_name_with_default(self, mock_add_client):
         test._regional_clients[sentinel.default_region] = sentinel.default_client
         client = test._client("")
         assert client is sentinel.default_client
-        mock_add_client.assert_called_once_with(sentinel.default_region)
+        mock_add_client.assert_called_with(sentinel.default_region)
 
     def test_client_no_region_name_without_default(self):
-        test = KMSMasterKeyProvider()
+        test = KMSMasterKeyProvider(botocore_session=self.botocore_no_region_session)
         with pytest.raises(UnknownRegionError) as excinfo:
             test._client("")
         excinfo.match("No default region found and no region determinable from key id: *")