diff --git a/rest_framework/authtoken/serializers.py b/rest_framework/authtoken/serializers.py
index e5f46dd66..580ef57c5 100644
--- a/rest_framework/authtoken/serializers.py
+++ b/rest_framework/authtoken/serializers.py
@@ -16,19 +16,17 @@ class AuthTokenSerializer(serializers.Serializer):
         username = attrs.get('username')
         password = attrs.get('password')
 
-        if username and password:
-            user = authenticate(request=self.context.get('request'),
-                                username=username, password=password)
 
-            # The authenticate call simply returns None for is_active=False
-            # users. (Assuming the default ModelBackend authentication
-            # backend.)
-            if not user:
-                msg = _('Unable to log in with provided credentials.')
-                raise serializers.ValidationError(msg, code='authorization')
-        else:
-            msg = _('Must include "username" and "password".')
+        user = authenticate(request=self.context.get('request'),
+                            username=username, password=password)
+
+        # The authenticate call simply returns None for is_active=False
+        # users. (Assuming the default ModelBackend authentication
+        # backend.)
+        if not user:
+            msg = _('Unable to log in with provided credentials.')
             raise serializers.ValidationError(msg, code='authorization')
 
+
         attrs['user'] = user
         return attrs