diff --git a/app/accept_invite/rest.py b/app/accept_invite/rest.py
index a6ac0773b..0485fc58a 100644
--- a/app/accept_invite/rest.py
+++ b/app/accept_invite/rest.py
@@ -35,8 +35,8 @@ def validate_invitation_token(invitation_type, token):
                                       max_age_seconds)
     except SignatureExpired:
         errors = {'invitation':
-                  ['Your invitation to GOV.UK Notify has expired. '
-                   'Please ask the person that invited you to send you another one']}
+                  'Your invitation to GOV.UK Notify has expired. '
+                  'Please ask the person that invited you to send you another one'}
         raise InvalidRequest(errors, status_code=400)
     except BadData:
         errors = {'invitation': 'Something’s wrong with this link. Make sure you’ve copied the whole thing.'}
diff --git a/tests/app/accept_invite/test_accept_invite_rest.py b/tests/app/accept_invite/test_accept_invite_rest.py
index f0c806473..a351eb4de 100644
--- a/tests/app/accept_invite/test_accept_invite_rest.py
+++ b/tests/app/accept_invite/test_accept_invite_rest.py
@@ -19,9 +19,9 @@ def test_validate_invitation_token_for_expired_token_returns_400(client, invitat
     assert response.status_code == 400
     json_resp = json.loads(response.get_data(as_text=True))
     assert json_resp['result'] == 'error'
-    assert json_resp['message'] == {'invitation': [
-        'Your invitation to GOV.UK Notify has expired. '
-        'Please ask the person that invited you to send you another one']}
+    assert json_resp['message'] == {
+        'invitation': 'Your invitation to GOV.UK Notify has expired. '
+                      'Please ask the person that invited you to send you another one'}
 
 
 @pytest.mark.parametrize('invitation_type', ['service', 'organisation'])