diff --git a/core/forms.py b/core/forms.py
index 9d02066..4530b0e 100644
--- a/core/forms.py
+++ b/core/forms.py
@@ -29,7 +29,7 @@ class CaveForm(ModelForm):
             self._errors["official_name"] = self.error_class(["This field is required when there is a kataster number."]) 
         if self.cleaned_data.get("area") == []:
             self._errors["area"] = self.error_class(["This field is required."])
-        if self.cleaned_data.get("url").startswith("/"):
+        if self.cleaned_data.get("url") and self.cleaned_data.get("url").startswith("/"):
             self._errors["url"] = self.error_class(["This field can not start with a /."])
         return self.cleaned_data