diff --git a/linkcheck/director/aggregator.py b/linkcheck/director/aggregator.py
index 6600b3e4..c8fdb175 100644
--- a/linkcheck/director/aggregator.py
+++ b/linkcheck/director/aggregator.py
@@ -83,19 +83,24 @@ class Aggregate:
         if not url:
             return
         user, password = self.config.get_user_password(url)
+        if not user and not password:
+            raise LinkCheckerError(
+                "loginurl is configured but neither user nor password are set")
         session = requests.Session()
         # XXX user-agent header
         # XXX timeout
         log.debug(LOG_CHECK, "Getting login form %s", url)
         response = session.get(url)
         response.raise_for_status()
-        cgiuser = self.config["loginuserfield"]
-        cgipassword = self.config["loginpasswordfield"]
+        cgiuser = self.config["loginuserfield"] if user else None
+        cgipassword = self.config["loginpasswordfield"] if password else None
         form = loginformsearch.search_form(response.text, cgiuser, cgipassword)
         if not form:
             raise LinkCheckerError("Login form not found at %s" % url)
-        form.data[cgiuser] = user
-        form.data[cgipassword] = password
+        if user:
+            form.data[cgiuser] = user
+        if password:
+            form.data[cgipassword] = password
         for key, value in self.config["loginextrafields"].items():
             form.data[key] = value
         formurl = parse.urljoin(url, form.url)
diff --git a/linkcheck/htmlutil/loginformsearch.py b/linkcheck/htmlutil/loginformsearch.py
index 0beb69e8..576bef01 100644
--- a/linkcheck/htmlutil/loginformsearch.py
+++ b/linkcheck/htmlutil/loginformsearch.py
@@ -41,7 +41,7 @@ def search_form(content, cgiuser, cgipassword):
     CGI fields. If no form is found return None.
     """
     soup = htmlsoup.make_soup(content)
-    cginames = {cgiuser, cgipassword}
+    cginames = {cgiuser, cgipassword} - {None}
     for form_element in soup.find_all("form", action=True):
         form = Form(form_element["action"])
         for input_element in form_element.find_all("input",