diff --git a/Featurecreep.md b/Featurecreep.md
index 970205465b..64ed3d75d0 100644
--- a/Featurecreep.md
+++ b/Featurecreep.md
@@ -217,7 +217,7 @@ Sorry for some of them being in German, I'll tranlate them at some point.
 * [x] Reminders should use an extra table so we can make reverse lookups aka "give me all tasks with reminders in this period" which we'll need for things like email reminders notifications
 * [x] When giving a user access to a list/namespace, they should be reffered to by uuid, not numeric id
 * [x] Adding users to a team should also use uuid
-* [ ] Check if the team/user really exist before updating them on lists/namespaces
+* [x] Check if the team/user really exist before updating them on lists/namespaces
 
 ### Linters
 
diff --git a/pkg/models/list_users_test.go b/pkg/models/list_users_test.go
index 2d3097d879..9d38a9e41e 100644
--- a/pkg/models/list_users_test.go
+++ b/pkg/models/list_users_test.go
@@ -218,7 +218,7 @@ func TestListUser_ReadAll(t *testing.T) {
 func TestListUser_Update(t *testing.T) {
 	type fields struct {
 		ID       int64
-		UserID   int64
+		Username string
 		ListID   int64
 		Right    Right
 		Created  int64
@@ -235,33 +235,33 @@ func TestListUser_Update(t *testing.T) {
 		{
 			name: "Test Update Normally",
 			fields: fields{
-				ListID: 3,
-				UserID: 1,
-				Right:  RightAdmin,
+				ListID:   3,
+				Username: "user1",
+				Right:    RightAdmin,
 			},
 		},
 		{
 			name: "Test Update to write",
 			fields: fields{
-				ListID: 3,
-				UserID: 1,
-				Right:  RightWrite,
+				ListID:   3,
+				Username: "user1",
+				Right:    RightWrite,
 			},
 		},
 		{
 			name: "Test Update to Read",
 			fields: fields{
-				ListID: 3,
-				UserID: 1,
-				Right:  RightRead,
+				ListID:   3,
+				Username: "user1",
+				Right:    RightRead,
 			},
 		},
 		{
 			name: "Test Update with invalid right",
 			fields: fields{
-				ListID: 3,
-				UserID: 1,
-				Right:  500,
+				ListID:   3,
+				Username: "user1",
+				Right:    500,
 			},
 			wantErr: true,
 			errType: IsErrInvalidRight,
@@ -271,7 +271,7 @@ func TestListUser_Update(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			lu := &ListUser{
 				ID:       tt.fields.ID,
-				UserID:   tt.fields.UserID,
+				Username: tt.fields.Username,
 				ListID:   tt.fields.ListID,
 				Right:    tt.fields.Right,
 				Created:  tt.fields.Created,
diff --git a/pkg/models/list_users_update.go b/pkg/models/list_users_update.go
index a69c5891dc..29bc6c8245 100644
--- a/pkg/models/list_users_update.go
+++ b/pkg/models/list_users_update.go
@@ -40,6 +40,13 @@ func (lu *ListUser) Update() (err error) {
 		return err
 	}
 
+	// Check if the user exists
+	user, err := GetUserByUsername(lu.Username)
+	if err != nil {
+		return err
+	}
+	lu.UserID = user.ID
+
 	_, err = x.
 		Where("list_id = ? AND user_id = ?", lu.ListID, lu.UserID).
 		Cols("right").
diff --git a/pkg/models/namespace_users_test.go b/pkg/models/namespace_users_test.go
index 29c65a8dcf..fefdf12f95 100644
--- a/pkg/models/namespace_users_test.go
+++ b/pkg/models/namespace_users_test.go
@@ -218,7 +218,7 @@ func TestNamespaceUser_ReadAll(t *testing.T) {
 func TestNamespaceUser_Update(t *testing.T) {
 	type fields struct {
 		ID          int64
-		UserID      int64
+		Username    string
 		NamespaceID int64
 		Right       Right
 		Created     int64
@@ -236,7 +236,7 @@ func TestNamespaceUser_Update(t *testing.T) {
 			name: "Test Update Normally",
 			fields: fields{
 				NamespaceID: 3,
-				UserID:      1,
+				Username:    "user1",
 				Right:       RightAdmin,
 			},
 		},
@@ -244,7 +244,7 @@ func TestNamespaceUser_Update(t *testing.T) {
 			name: "Test Update to write",
 			fields: fields{
 				NamespaceID: 3,
-				UserID:      1,
+				Username:    "user1",
 				Right:       RightWrite,
 			},
 		},
@@ -252,7 +252,7 @@ func TestNamespaceUser_Update(t *testing.T) {
 			name: "Test Update to Read",
 			fields: fields{
 				NamespaceID: 3,
-				UserID:      1,
+				Username:    "user1",
 				Right:       RightRead,
 			},
 		},
@@ -260,7 +260,7 @@ func TestNamespaceUser_Update(t *testing.T) {
 			name: "Test Update with invalid right",
 			fields: fields{
 				NamespaceID: 3,
-				UserID:      1,
+				Username:    "user1",
 				Right:       500,
 			},
 			wantErr: true,
@@ -271,7 +271,7 @@ func TestNamespaceUser_Update(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			nu := &NamespaceUser{
 				ID:          tt.fields.ID,
-				UserID:      tt.fields.UserID,
+				Username:    tt.fields.Username,
 				NamespaceID: tt.fields.NamespaceID,
 				Right:       tt.fields.Right,
 				Created:     tt.fields.Created,
diff --git a/pkg/models/namespace_users_update.go b/pkg/models/namespace_users_update.go
index 82273f1f62..bcaa55d142 100644
--- a/pkg/models/namespace_users_update.go
+++ b/pkg/models/namespace_users_update.go
@@ -40,6 +40,13 @@ func (nu *NamespaceUser) Update() (err error) {
 		return err
 	}
 
+	// Check if the user exists
+	user, err := GetUserByUsername(nu.Username)
+	if err != nil {
+		return err
+	}
+	nu.UserID = user.ID
+
 	_, err = x.
 		Where("namespace_id = ? AND user_id = ?", nu.NamespaceID, nu.UserID).
 		Cols("right").