diff --git a/app/main/forms.py b/app/main/forms.py
index f6f7412a2..2c89859a4 100644
--- a/app/main/forms.py
+++ b/app/main/forms.py
@@ -40,7 +40,14 @@ from wtforms import (
     ValidationError,
     validators,
 )
-from wtforms.validators import URL, DataRequired, Length, Optional, Regexp
+from wtforms.validators import (
+    URL,
+    DataRequired,
+    InputRequired,
+    Length,
+    Optional,
+    Regexp,
+)
 
 from app.formatters import format_thousands, guess_name_from_email_address
 from app.main.validators import (
@@ -1300,7 +1307,7 @@ class FreeSMSAllowance(StripWhitespaceForm):
     free_sms_allowance = GovukIntegerField(
         'Numbers of text message fragments per year',
         validators=[
-            DataRequired(message='Cannot be empty')
+            InputRequired(message='Cannot be empty')
         ]
     )
 
diff --git a/tests/app/main/views/test_service_settings.py b/tests/app/main/views/test_service_settings.py
index 1ead2e1f4..9d22a36ec 100644
--- a/tests/app/main/views/test_service_settings.py
+++ b/tests/app/main/views/test_service_settings.py
@@ -3800,6 +3800,7 @@ def test_should_show_page_to_set_sms_allowance(
 
 @freeze_time("2017-04-01 11:09:00.061258")
 @pytest.mark.parametrize('given_allowance, expected_api_argument', [
+    ('0', 0),
     ('1', 1),
     ('250000', 250000),
     pytest.param('foo', 'foo', marks=pytest.mark.xfail),