diff --git a/src/main/java/com/wire/bots/hold/utils/Collector.java b/src/main/java/com/wire/bots/hold/utils/Collector.java
index 3ab7814..ab54e64 100644
--- a/src/main/java/com/wire/bots/hold/utils/Collector.java
+++ b/src/main/java/com/wire/bots/hold/utils/Collector.java
@@ -68,7 +68,7 @@ public void addSystem(String text, String dateTime, String type) throws ParseExc
 
     private Sender sender(User user, Message message) {
         Sender sender = new Sender();
-        sender.senderId = user.id;
+        sender.senderId = user.id.toString();
         sender.name = user.name;
         sender.accent = toColor(user.accent);
         sender.avatar = getAvatar(user);
@@ -79,7 +79,7 @@ private Sender sender(User user, Message message) {
     private Sender system(Message message, String type) {
         Sender sender = new Sender();
         sender.system = "system";
-        sender.senderId = new QualifiedId(UUID.randomUUID(), null); // TODO(WPB-11287): Change null to default domain
+        sender.senderId = "system";
         sender.avatar = systemIcon(type);
         sender.messages.add(message);
         return sender;
@@ -213,7 +213,7 @@ public static class Message {
     }
 
     public static class Sender {
-        QualifiedId senderId;
+        String senderId;
         String avatar;
         String name;
         String accent;
diff --git a/src/main/resources/db/migration/V108__add_domain_column_in_access_events.sql b/src/main/resources/db/migration/V108__add_domain_column_in_access_events.sql
index db3da8e..256caab 100644
--- a/src/main/resources/db/migration/V108__add_domain_column_in_access_events.sql
+++ b/src/main/resources/db/migration/V108__add_domain_column_in_access_events.sql
@@ -1,6 +1,9 @@
 ALTER TABLE Access
 ADD COLUMN userDomain VARCHAR(255) DEFAULT null;
 
+-- Dropping the PRIMARY KEY and adding a UNIQUE constraint with both Id and Domain,
+-- because Postgresql doesn't like a nullable fields being PRIMARY KEY.
+-- (previous tentative: PRIMARY KEY (userId, userDomain)
 ALTER TABLE Access
 DROP CONSTRAINT IF EXISTS access_pkey;