diff --git a/core/forms.py b/core/forms.py
index 0d03d19..b27a8a8 100644
--- a/core/forms.py
+++ b/core/forms.py
@@ -218,8 +218,9 @@ class EntranceForm(ModelForm):
         )
 
     def clean(self):
-        if self.cleaned_data.get("url").startswith("/"):
-            self._errors["url"] = self.error_class(["This field cannot start with a /."])
+        if self.cleaned_data.get("url"):
+            if self.cleaned_data.get("url").startswith("/"):
+                self._errors["url"] = self.error_class(["This field cannot start with a /."])
         return self.cleaned_data