diff --git a/django_etesync/serializers.py b/django_etesync/serializers.py
index f117b31..337f695 100644
--- a/django_etesync/serializers.py
+++ b/django_etesync/serializers.py
@@ -381,13 +381,7 @@ class AuthenticationSignupSerializer(serializers.Serializer):
 
 
 class AuthenticationLoginChallengeSerializer(serializers.Serializer):
-    username = serializers.CharField(required=False)
-    email = serializers.EmailField(required=False)
-
-    def validate(self, data):
-        if not data.get('email') and not data.get('username'):
-            raise serializers.ValidationError('Either email or username must be set')
-        return data
+    username = serializers.CharField(required=True)
 
     def create(self, validated_data):
         raise NotImplementedError()
diff --git a/django_etesync/views.py b/django_etesync/views.py
index 52d3531..ac7a007 100644
--- a/django_etesync/views.py
+++ b/django_etesync/views.py
@@ -579,17 +579,9 @@ class AuthenticationViewSet(viewsets.ViewSet):
         data = self.login_response_data(user)
         return Response(data, status=status.HTTP_201_CREATED)
 
-    def get_login_user(self, serializer):
-        username = serializer.validated_data.get('username')
-        email = serializer.validated_data.get('email')
-        if username:
-            kwargs = {User.USERNAME_FIELD: username}
-            user = get_object_or_404(self.get_queryset(), **kwargs)
-        elif email:
-            kwargs = {User.EMAIL_FIELD: email}
-            user = get_object_or_404(self.get_queryset(), **kwargs)
-
-        return user
+    def get_login_user(self, username):
+        kwargs = {User.USERNAME_FIELD: username}
+        return get_object_or_404(self.get_queryset(), **kwargs)
 
     @action_decorator(detail=False, methods=['POST'])
     def login_challenge(self, request):
@@ -597,7 +589,8 @@ class AuthenticationViewSet(viewsets.ViewSet):
 
         serializer = AuthenticationLoginChallengeSerializer(data=request.data)
         if serializer.is_valid():
-            user = self.get_login_user(serializer)
+            username = serializer.validated_data.get('username')
+            user = self.get_login_user(username)
 
             salt = bytes(user.userinfo.salt)
             enc_key = self.get_encryption_key(salt)
@@ -631,7 +624,8 @@ class AuthenticationViewSet(viewsets.ViewSet):
 
             serializer = AuthenticationLoginInnerSerializer(data=response, context={'host': request.get_host()})
             if serializer.is_valid():
-                user = self.get_login_user(serializer)
+                username = serializer.validated_data.get('username')
+                user = self.get_login_user(username)
                 host = serializer.validated_data['host']
                 challenge = serializer.validated_data['challenge']