diff --git a/wagtail/wagtailimages/forms.py b/wagtail/wagtailimages/forms.py
index 27bd3c725..46bdd997d 100644
--- a/wagtail/wagtailimages/forms.py
+++ b/wagtail/wagtailimages/forms.py
@@ -57,4 +57,4 @@ class URLGeneratorForm(forms.Form):
     )
     width = forms.IntegerField(_("Width"), min_value=0)
     height = forms.IntegerField(_("Height"), min_value=0)
-    closeness = forms.IntegerField(_("Closeness"), min_value=0)
+    closeness = forms.IntegerField(_("Closeness"), min_value=0, initial=0)