diff --git a/CHANGES.txt b/CHANGES.txt
index e372e7f..6170d97 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -26,6 +26,9 @@ Changes
 - Drop support for Python 2.7 and Python 3.4.
   [aleksihakli]
 
+- Drop old single-argument signature format for ``AXES_USERNAME_CALLABLE``.
+  [aleksihakli]
+
 
 4.5.4 (2019-01-15)
 ------------------
diff --git a/axes/tests/test_utils.py b/axes/tests/test_utils.py
index cc2eb66..03732c7 100644
--- a/axes/tests/test_utils.py
+++ b/axes/tests/test_utils.py
@@ -172,22 +172,9 @@ class UtilsTest(TestCase):
 
         self.assertEqual(expected_in_credentials, actual)
 
-    def sample_customize_username(request):
+    def sample_customize_username(request, credentials):
         return 'prefixed-' + request.POST.get('username')
 
-    @override_settings(AXES_USERNAME_FORM_FIELD='username')
-    @override_settings(AXES_USERNAME_CALLABLE=sample_customize_username)
-    def test_custom_get_client_username(self):
-        provided = 'test-username'
-        expected = 'prefixed-' + provided
-
-        request = HttpRequest()
-        request.POST['username'] = provided
-
-        actual = get_client_username(request)
-
-        self.assertEqual(expected, actual)
-
     @override_settings(AXES_USERNAME_FORM_FIELD='username')
     @override_settings(AXES_USERNAME_CALLABLE=sample_customize_username)
     def test_custom_get_client_username_from_request(self):
@@ -223,18 +210,30 @@ class UtilsTest(TestCase):
 
         self.assertEqual(expected_in_credentials, actual)
 
-    def sample_get_client_username_too_few_arguments():
+    def sample_get_client_username(request, credentials):
+        return 'example'
+
+    @override_settings(AXES_USERNAME_CALLABLE=sample_get_client_username)
+    def test_get_client_username(self):
+        self.assertEqual('example', get_client_username(HttpRequest(), {}))
+
+    @override_settings(AXES_USERNAME_CALLABLE=sample_get_client_username)
+    def test_get_client_username_too_many_arguments(self):
+        with self.assertRaises(TypeError):
+            actual = get_client_username(HttpRequest(), {}, None)
+
+    def sample_get_client_username_too_few_arguments(request):
         pass
 
     @override_settings(AXES_USERNAME_CALLABLE=sample_get_client_username_too_few_arguments)
-    def test_get_client_username_too_few_arguments_invalid_callable(self):
+    def test_get_client_username_invalid_callable_too_few_arguments(self):
         with self.assertRaises(TypeError):
             actual = get_client_username(HttpRequest(), {})
 
-    def sample_get_client_username_too_many_arguments(one, two, three):
+    def sample_get_client_username_too_many_arguments(request, credentials, extra_argument):
         pass
 
     @override_settings(AXES_USERNAME_CALLABLE=sample_get_client_username_too_many_arguments)
-    def test_get_client_username_too_many_arguments_invalid_callable(self):
+    def test_get_client_username_invalid_callable_too_many_arguments(self):
         with self.assertRaises(TypeError):
             actual = get_client_username(HttpRequest(), {})
diff --git a/axes/utils.py b/axes/utils.py
index fd93395..b4df189 100644
--- a/axes/utils.py
+++ b/axes/utils.py
@@ -1,5 +1,4 @@
 from datetime import timedelta
-from inspect import getargspec
 from logging import getLogger
 from socket import error, inet_pton, AF_INET6
 
@@ -76,30 +75,17 @@ def get_client_username(request, credentials=None):
 
     The order of preference for fetching the username is as follows:
 
-    1. If configured, use `AXES_USERNAME_CALLABLE`, and supply either `request` or `request, credentials` as arguments
-       depending on the function argument count (multiple signatures are supported for backwards compatibility)
-    2. If given, use `credentials` and fetch username from `AXES_USERNAME_FORM_FIELD` (defaults to `username`)
-    3. Use request.POST and fetch username from `AXES_USERNAME_FORM_FIELD` (defaults to `username`)
+    1. If configured, use ``AXES_USERNAME_CALLABLE``, and supply ``request, credentials`` as arguments
+    2. If given, use ``credentials`` and fetch username from ``AXES_USERNAME_FORM_FIELD`` (defaults to ``username``)
+    3. Use request.POST and fetch username from ``AXES_USERNAME_FORM_FIELD`` (defaults to ``username``)
 
-    :param request: incoming Django `HttpRequest` or similar object from authentication backend or other source
-    :param credentials: incoming credentials `dict` or similar object from authentication backend or other source
+    :param request: incoming Django ``HttpRequest`` or similar object from authentication backend or other source
+    :param credentials: incoming credentials ``dict`` or similar object from authentication backend or other source
     """
 
     if settings.AXES_USERNAME_CALLABLE:
-        num_args = len(
-            getargspec(settings.AXES_USERNAME_CALLABLE).args  # pylint: disable=deprecated-method
-        )
-
-        if num_args == 2:
-            logger.debug('Using AXES_USERNAME_CALLABLE for username with two arguments: request, credentials')
-            return settings.AXES_USERNAME_CALLABLE(request, credentials)
-
-        if num_args == 1:
-            logger.debug('Using AXES_USERNAME_CALLABLE for username with one argument: request')
-            return settings.AXES_USERNAME_CALLABLE(request)
-
-        logger.error('Using AXES_USERNAME_CALLABLE for username failed: wrong number of arguments %s', num_args)
-        raise TypeError('Wrong number of arguments in function call to AXES_USERNAME_CALLABLE', num_args)
+        logger.debug('Using AXES_USERNAME_CALLABLE to get username')
+        return settings.AXES_USERNAME_CALLABLE(request, credentials)
 
     if credentials:
         logger.debug('Using `credentials` to get username with key AXES_USERNAME_FORM_FIELD')
diff --git a/docs/configuration.rst b/docs/configuration.rst
index b83c1ef..9050ceb 100644
--- a/docs/configuration.rst
+++ b/docs/configuration.rst
@@ -145,8 +145,8 @@ Use the ``python manage.py check`` command to verify the correct configuration i
 development and production environments. It is probably best to use this step as part
 of your regular CI workflows to verify that your project is not misconfigured.
 
-django-axes uses the checks to verify your cache configuration to see that your caches
-should be functional with the configuration axes. Many people have different configurations
+Axes uses the checks to verify your cache configuration to see that your caches
+should be functional with the configuration of Axes. Many people have different configurations
 for their development and production environments.
 
 
@@ -185,7 +185,7 @@ add an extra cache to ``CACHES`` with a name of your choice::
         }
     }
 
-The next step is to tell axes to use this cache through adding ``AXES_CACHE``
+The next step is to tell Axes to use this cache through adding ``AXES_CACHE``
 to your ``settings.py`` file::
 
     AXES_CACHE = 'axes_cache'
@@ -221,7 +221,7 @@ Customizing Axes
 You have a couple options available to you to customize ``django-axes`` a bit.
 These should be defined in your ``settings.py`` file.
 
-* ``AXES_CACHE``: The name of the cache for axes to use.
+* ``AXES_CACHE``: The name of the cache for Axes to use.
   Default: ``'default'``
 * ``AXES_FAILURE_LIMIT``: The number of login attempts allowed before a
   record is created for the failed logins.  Default: ``3``
@@ -235,7 +235,7 @@ These should be defined in your ``settings.py`` file.
   old failed login attempts will be forgotten. Can be set to a python
   timedelta object or an integer. If an integer, will be interpreted as a
   number of hours.  Default: ``None``
-* ``AXES_LOGGER``: If set, specifies a logging mechanism for axes to use.
+* ``AXES_LOGGER``: If set, specifies a logging mechanism for Axes to use.
   Default: ``'axes.watch_login'``
 * ``AXES_LOCKOUT_TEMPLATE``: If set, specifies a template to render when a
   user is locked out. Template receives cooloff_time and failure_limit as
@@ -247,11 +247,11 @@ These should be defined in your ``settings.py`` file.
   Default: ``True``
 * ``AXES_USERNAME_FORM_FIELD``: the name of the form field that contains your
   users usernames. Default: ``username``
-* ``AXES_USERNAME_CALLABLE``: A callable function that takes either one or two arguments:
-  ``AXES_USERNAME_CALLABLE(request)`` or ``AXES_USERNAME_CALLABLE(request, credentials)``.
+* ``AXES_USERNAME_CALLABLE``: A callable function that takes two arguments:
+  ``AXES_USERNAME_CALLABLE(request, credentials)``.
   The ``request`` is a HttpRequest like object and the ``credentials`` is a dictionary like object.
   ``credentials`` are the ones that were passed to Django ``authenticate()`` in the login flow.
-  If no function is supplied, axes fetches the username from the ``credentials`` or ``request.POST``
+  If no function is supplied, Axes fetches the username from the ``credentials`` or ``request.POST``
   dictionaries based on ``AXES_USERNAME_FORM_FIELD``. Default: ``None``
 * ``AXES_PASSWORD_FORM_FIELD``: the name of the form or credentials field that contains your
   users password. Default: ``password``
diff --git a/docs/migration.rst b/docs/migration.rst
index d26fafd..1f292f0 100644
--- a/docs/migration.rst
+++ b/docs/migration.rst
@@ -33,3 +33,5 @@ API endpoints or names for compatibility reasons.
   ``axes.exceptions.AxesSignalPermissionDenied`` on lockouts.
 - ``AxesMiddleware`` was added to return lockout responses.
   The middleware handles ``axes.exception.AxesSignalPermissionDenied``.
+- ``AXES_USERNAME_CALLABLE`` is now always called with two arguments,
+  ``request`` and ``credentials`` instead of ``request``.
diff --git a/docs/usage.rst b/docs/usage.rst
index 9c13e84..11b24ac 100644
--- a/docs/usage.rst
+++ b/docs/usage.rst
@@ -21,7 +21,7 @@ Once ``axes`` is in your ``INSTALLED_APPS`` in your project settings file, you c
 login and logout of your application via the ``django.contrib.auth`` views.
 The attempts will be logged and visible in the "Access Attempts" section in admin.
 
-By default, django-axes will lock out repeated access attempts from the same IP address.
+By default, Axes will lock out repeated access attempts from the same IP address.
 You can allow this IP to attempt again by deleting relevant ``AccessAttempt`` records.
 
 Records can be deleted, for example, by using the Django admin application.