diff --git a/www/conservancy/apps/assignment/forms.py b/www/conservancy/apps/assignment/forms.py
index 8777e0b4857fc36018b98894832587f60447ed31..288dc75aa54bcfee7bf8e7da40aad497b97e6328 100644
--- a/www/conservancy/apps/assignment/forms.py
+++ b/www/conservancy/apps/assignment/forms.py
@@ -62,3 +62,6 @@ class AssignmentForm(forms.ModelForm):
     def clean_period_ends(self):
         if 'period_begins' in self.cleaned_data and 'period_ends' in self.cleaned_data and self.cleaned_data['period_begins'] and self.cleaned_data['period_ends'] and self.cleaned_data['period_begins'] > self.cleaned_data['period_ends']:
             raise ValidationError('End of period is before start')
+
+        if self.cleaned_data['period_end_type'] == 'a specific past date' and not self.cleaned_data['period_ends']:
+            raise ValidationError('This field is required')