diff --git a/app/main/views/register.py b/app/main/views/register.py
index 9e92fa444..f94895695 100644
--- a/app/main/views/register.py
+++ b/app/main/views/register.py
@@ -43,10 +43,11 @@ def register():
 @main.route('/register-from-invite', methods=['GET', 'POST'])
 def register_from_invite():
     invited_user = session.get('invited_user')
-    form = RegisterUserFromInviteForm()
     if not invited_user:
         abort(404)
 
+    form = RegisterUserFromInviteForm()
+
     if form.validate_on_submit():
         if form.service.data != invited_user['service'] or form.email_address.data != invited_user['email_address']:
             abort(400)
@@ -65,11 +66,11 @@ def register_from_invite():
     return render_template('views/register-from-invite.html', email_address=invited_user['email_address'], form=form)
 
 
-def _do_registration(form, service=None, send_sms=True, send_email=True):
+def _do_registration(form, send_sms=True, send_email=True):
     if user_api_client.is_email_unique(form.email_address.data):
         user = user_api_client.register_user(form.name.data,
                                              form.email_address.data,
-                                             form.mobile_number.data,
+                                             form.mobile_number.data or None,
                                              form.password.data,
                                              form.auth_type.data)
 
diff --git a/tests/app/main/views/test_register.py b/tests/app/main/views/test_register.py
index a366e5282..544e69d8c 100644
--- a/tests/app/main/views/test_register.py
+++ b/tests/app/main/views/test_register.py
@@ -8,6 +8,7 @@ from flask import (
     url_for,
     session
 )
+from flask_login import current_user
 from app.notify_client.models import InvitedUser
 
 
@@ -165,7 +166,7 @@ def test_register_with_existing_email_sends_emails(
     assert response.location == url_for('main.registration_continue', _external=True)
 
 
-def test_register_from_invite_(
+def test_register_from_invite(
     client,
     fake_uuid,
     mock_is_email_unique,
@@ -224,3 +225,105 @@ def test_register_from_invite_when_user_registers_in_another_browser(
     )
     assert response.status_code == 302
     assert response.location == url_for('main.verify', _external=True)
+
+
+def test_register_from_email_auth_invite(
+    client,
+    sample_invite,
+    mock_is_email_unique,
+    mock_register_user,
+    mock_get_user,
+    mock_send_verify_email,
+    mock_send_verify_code,
+    mock_accept_invite,
+):
+    sample_invite['auth_type'] = 'email_auth'
+    with client.session_transaction() as session:
+        session['invited_user'] = sample_invite
+    assert not current_user.is_authenticated
+
+    data = {
+        'name': 'invited user',
+        'email_address': sample_invite['email_address'],
+        'mobile_number': '07700900001',
+        'password': 'FSLKAJHFNvdzxgfyst',
+        'service': sample_invite['service'],
+        'auth_type': 'email_auth',
+    }
+
+    resp = client.post(url_for('main.register_from_invite'), data=data)
+    assert resp.status_code == 302
+    assert resp.location == url_for('main.add_service', first='first', _external=True)
+
+    # doesn't send any 2fa code
+    assert not mock_send_verify_email.called
+    assert not mock_send_verify_code.called
+    # creates user with email_auth set
+    mock_register_user.assert_called_once_with(
+        data['name'],
+        data['email_address'],
+        data['mobile_number'],
+        data['password'],
+        data['auth_type']
+    )
+    mock_accept_invite.assert_called_once_with(sample_invite['service'], sample_invite['id'])
+    # just logs them in
+    assert current_user.is_authenticated
+
+    with client.session_transaction() as session:
+        # invited user details are still there so they can get added to the service
+        assert session['invited_user'] == sample_invite
+
+
+def test_can_register_email_auth_without_phone_number(
+    client,
+    sample_invite,
+    mock_is_email_unique,
+    mock_register_user,
+    mock_get_user,
+    mock_send_verify_email,
+    mock_send_verify_code,
+    mock_accept_invite,
+):
+    sample_invite['auth_type'] = 'email_auth'
+    with client.session_transaction() as session:
+        session['invited_user'] = sample_invite
+
+    data = {
+        'name': 'invited user',
+        'email_address': sample_invite['email_address'],
+        'mobile_number': '',
+        'password': 'FSLKAJHFNvdzxgfyst',
+        'service': sample_invite['service'],
+        'auth_type': 'email_auth'
+    }
+
+    resp = client.post(url_for('main.register_from_invite'), data=data)
+    assert resp.status_code == 302
+    assert resp.location == url_for('main.add_service', first='first', _external=True)
+
+    mock_register_user.assert_called_once_with(
+        ANY,
+        ANY,
+        None,  # mobile_number
+        ANY,
+        ANY
+    )
+
+
+def test_cannot_register_with_sms_auth_and_missing_mobile_number(
+    client,
+    mock_send_verify_code,
+    mock_get_user_by_email_not_found,
+    mock_login,
+):
+    response = client.post(url_for('main.register'),
+                           data={'name': 'Missing Mobile',
+                                 'email_address': 'missing_mobile@example.gov.uk',
+                                 'password': 'validPassword!'})
+
+    assert response.status_code == 200
+    page = BeautifulSoup(response.data.decode('utf-8'), 'html.parser')
+    err = page.select_one('.error-message')
+    assert err.text.strip() == 'Can’t be empty'
+    assert err.attrs['data-error-label'] == 'mobile_number'