diff --git a/taiga/export_import/serializers.py b/taiga/export_import/serializers.py
index cb2ea35d..807d8d42 100644
--- a/taiga/export_import/serializers.py
+++ b/taiga/export_import/serializers.py
@@ -72,6 +72,24 @@ class UserRelatedField(serializers.RelatedField):
             return None
 
 
+class UserPkField(serializers.RelatedField):
+    read_only = False
+
+    def to_native(self, obj):
+        try:
+            user = users_models.User.objects.get(pk=obj)
+            return user.email
+        except users_models.User.DoesNotExist:
+            return None
+
+    def from_native(self, data):
+        try:
+            user = users_models.User.objects.get(email=data)
+            return user.pk
+        except users_models.User.DoesNotExist:
+            return None
+
+
 class CommentField(serializers.WritableField):
     read_only = False
 
@@ -112,10 +130,10 @@ class HistoryUserField(JsonField):
 
     def from_native(self, data):
         if data is None:
-            return []
+            return {}
 
         if len(data) < 2:
-            return []
+            return {}
 
         return {"pk": UserRelatedField().from_native(data[0]).pk, "name": data[1]}
 
@@ -125,14 +143,14 @@ class HistoryValuesField(JsonField):
         if obj is None:
             return []
         if "users" in obj:
-            obj['users'] = map(HistoryUserField().to_native, obj['users'])
+            obj['users'] = list(map(UserPkField().to_native, obj['users']))
         return obj
 
     def from_native(self, data):
         if data is None:
             return []
         if "users" in data:
-            data['users'] = map(HistoryUserField().from_native, data['users'])
+            data['users'] = list(map(UserPkField().from_native, data['users']))
         return data
 
 
@@ -140,15 +158,18 @@ class HistoryDiffField(JsonField):
     def to_native(self, obj):
         if obj is None:
             return []
+
         if "assigned_to" in obj:
-            obj['assigned_to'] = HistoryUserField().to_native(obj['assigned_to'])
+            obj['assigned_to'] = list(map(UserPkField().to_native, obj['assigned_to']))
+
         return obj
 
     def from_native(self, data):
         if data is None:
             return []
+
         if "assigned_to" in data:
-            data['assigned_to'] = HistoryUserField().from_native(data['assigned_to'])
+            data['assigned_to'] = list(map(UserPkField().from_native, data['assigned_to']))
         return data
 
 
@@ -289,6 +310,7 @@ class TaskExportSerializer(HistoryExportSerializerMixin, AttachmentExportSeriali
 class UserStoryExportSerializer(HistoryExportSerializerMixin, AttachmentExportSerializerMixin, serializers.ModelSerializer):
     role_points = RolePointsExportSerializer(many=True, required=False)
     owner = UserRelatedField(required=False)
+    assigned_to = UserRelatedField(required=False)
     status = ProjectRelatedField(slug_field="name")
     milestone = ProjectRelatedField(slug_field="name", required=False)
     watchers = UserRelatedField(many=True, required=False)
diff --git a/taiga/projects/userstories/models.py b/taiga/projects/userstories/models.py
index 8aa7c007..8c1b3020 100644
--- a/taiga/projects/userstories/models.py
+++ b/taiga/projects/userstories/models.py
@@ -154,6 +154,8 @@ class UserStory(OCCModelMixin, WatchedModelMixin, BlockedMixin, TaggedMixin, mod
 @receiver(models.signals.post_save, sender=UserStory,
           dispatch_uid="user_story_create_role_points_handler")
 def us_create_role_points_handler(sender, instance, **kwargs):
+    if instance._importing:
+        return
     instance.project.update_role_points(user_stories=[instance])