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",
diff --git a/tests/checker/data/loginform_password.html b/tests/checker/data/loginform_password.html
new file mode 100644
index 00000000..0fe9f6a4
--- /dev/null
+++ b/tests/checker/data/loginform_password.html
@@ -0,0 +1,3 @@
+<form action="/tests/checker/cgi-bin/input2cookies.py">
+<input name="password">
+</form>
diff --git a/tests/checker/data/loginform_user.html b/tests/checker/data/loginform_user.html
new file mode 100644
index 00000000..d12a02ec
--- /dev/null
+++ b/tests/checker/data/loginform_user.html
@@ -0,0 +1,3 @@
+<form action="/tests/checker/cgi-bin/input2cookies.py">
+<input name="login">
+</form>
diff --git a/tests/checker/test_loginurl.py b/tests/checker/test_loginurl.py
index ea38c206..dc43e74f 100644
--- a/tests/checker/test_loginurl.py
+++ b/tests/checker/test_loginurl.py
@@ -29,18 +29,36 @@ class TestLoginUrl(HttpServerTest):
         super().__init__(methodName=methodName)
         self.handler = CGIHandler
 
-    def test_loginurl(self):
+    def visit_loginurl(self, page, user=None, password=None, extrafields=False):
         confargs = {}
-        confargs["loginurl"] = self.get_url("loginform.html")
-        confargs["loginextrafields"] = {"extra_field": "default"}
+        confargs["loginurl"] = self.get_url(page)
+        if extrafields:
+            confargs["loginextrafields"] = {"extra_field": "default"}
         confargs["authentication"] = [{
-            "user": "test_user", "password": "test_password",
+            "user": user, "password": password,
             "pattern": re.compile("^http://localhost.*")
         }]
 
         aggregate = get_test_aggregate(confargs, {"expected": ""})
         aggregate.visit_loginurl()
 
-        self.assertEqual(aggregate.cookies["login"], "test_user")
-        self.assertEqual(aggregate.cookies["password"], "test_password")
-        self.assertEqual(aggregate.cookies["extra_field"], "default")
+        return aggregate.cookies
+
+    def test_loginurl(self):
+        cookies = self.visit_loginurl("loginform.html", "test_user",
+                                      "test_password", True)
+
+        self.assertEqual(cookies["login"], "test_user")
+        self.assertEqual(cookies["password"], "test_password")
+        self.assertEqual(cookies["extra_field"], "default")
+
+    def test_loginurl_user(self):
+        cookies = self.visit_loginurl("loginform_user.html", "test_user")
+
+        self.assertEqual(cookies["login"], "test_user")
+
+    def test_login_password(self):
+        cookies = self.visit_loginurl("loginform_password.html",
+                                      password="test_password")
+
+        self.assertEqual(cookies["password"], "test_password")