diff --git a/api/src/user/controllers/auth.controller.spec.ts b/api/src/user/controllers/auth.controller.spec.ts
index b35a5db7..15858d6e 100644
--- a/api/src/user/controllers/auth.controller.spec.ts
+++ b/api/src/user/controllers/auth.controller.spec.ts
@@ -137,6 +137,7 @@ describe('AuthController', () => {
       first_name: 'test',
       last_name: 'test',
       roles: [role!.id],
+      avatar: null,
     };
     await invitationService.create(baseUser);
   });
@@ -157,6 +158,7 @@ describe('AuthController', () => {
         email: 'test@test.test',
         password: 'test',
         roles: ['invalid role value'],
+        avatar: null,
       };
 
       await expect(authController.signup(userCreateDto)).rejects.toThrow(
@@ -174,6 +176,7 @@ describe('AuthController', () => {
         email: 'test@test.test',
         password: 'test',
         roles: ['659564cb4aa383c0d0dbc688'],
+        avatar: null,
       };
       const result = await authController.signup(userCreateDto);
       expect(userService.create).toHaveBeenCalledWith(userCreateDto);
diff --git a/api/src/user/controllers/user.controller.spec.ts b/api/src/user/controllers/user.controller.spec.ts
index b4c82fb3..c7f5d889 100644
--- a/api/src/user/controllers/user.controller.spec.ts
+++ b/api/src/user/controllers/user.controller.spec.ts
@@ -210,6 +210,7 @@ describe('UserController', () => {
         email: 'test@test.test',
         password: 'test',
         roles: [role!.id],
+        avatar: null,
       };
       const result = await userController.create(userDto);
       expect(userService.create).toHaveBeenCalledWith(userDto);
diff --git a/api/src/user/controllers/user.controller.ts b/api/src/user/controllers/user.controller.ts
index f765b58e..5f8ff25d 100644
--- a/api/src/user/controllers/user.controller.ts
+++ b/api/src/user/controllers/user.controller.ts
@@ -250,7 +250,7 @@ export class ReadWriteUserController extends ReadOnlyUserController {
           .map((role) => role.id),
         avatar: user.avatar
           ? (await this.attachmentService.findOne(user.avatar))?.id
-          : undefined,
+          : null,
       },
     });
     return await this.userService.create(user);
diff --git a/api/src/user/dto/user.dto.ts b/api/src/user/dto/user.dto.ts
index 56d298ac..2b082b6e 100644
--- a/api/src/user/dto/user.dto.ts
+++ b/api/src/user/dto/user.dto.ts
@@ -60,7 +60,7 @@ export class UserCreateDto {
   @IsOptional()
   @IsString()
   @IsObjectId({ message: 'Avatar must be a valid ObjectId' })
-  avatar?: string;
+  avatar: string | null = null;
 }
 
 export class UserEditProfileDto extends OmitType(PartialType(UserCreateDto), [
diff --git a/api/src/user/passport/session.serializer.ts b/api/src/user/passport/session.serializer.ts
index 3480cf0e..9db750b4 100644
--- a/api/src/user/passport/session.serializer.ts
+++ b/api/src/user/passport/session.serializer.ts
@@ -35,6 +35,6 @@ export class AuthSerializer extends PassportSerializer {
     done: (err: Error | null, user: SessionUser | null) => void,
   ) {
     const user = payload.id ? await this.userService.findOne(payload.id) : null;
-    user ? done(null, user) : done(null, null);
+    done(null, user);
   }
 }
diff --git a/api/src/user/schemas/user.schema.ts b/api/src/user/schemas/user.schema.ts
index 502036b1..a97d2057 100644
--- a/api/src/user/schemas/user.schema.ts
+++ b/api/src/user/schemas/user.schema.ts
@@ -91,7 +91,7 @@ export class UserStub extends BaseSchema {
     ref: 'Attachment',
     default: null,
   })
-  avatar?: unknown;
+  avatar: unknown;
 
   @Prop({
     type: String,
@@ -112,7 +112,7 @@ export class User extends UserStub {
   roles: string[];
 
   @Transform(({ obj }) => obj.avatar?.toString() || null)
-  avatar?: string | null;
+  avatar: string | null;
 }
 
 @Schema({ timestamps: true })
diff --git a/api/src/utils/test/fixtures/user.ts b/api/src/utils/test/fixtures/user.ts
index c5f9d5d9..55a1aaff 100644
--- a/api/src/utils/test/fixtures/user.ts
+++ b/api/src/utils/test/fixtures/user.ts
@@ -9,7 +9,7 @@
 import mongoose from 'mongoose';
 
 import { UserCreateDto } from '@/user/dto/user.dto';
-import { UserModel, User } from '@/user/schemas/user.schema';
+import { User, UserModel } from '@/user/schemas/user.schema';
 import { hash } from '@/user/utilities/bcryptjs';
 
 import { getFixturesWithDefaultValues } from '../defaultValues';
@@ -25,6 +25,7 @@ export const users: UserCreateDto[] = [
     email: 'admin@admin.admin',
     password: 'adminadmin',
     roles: ['0', '1'],
+    avatar: null,
   },
 ];
 
@@ -34,7 +35,6 @@ export const userDefaultValues: TFixturesDefaultValues<User> = {
   timezone: 'Europe/Berlin',
   sendEmail: false,
   resetCount: 0,
-  avatar: null,
 };
 
 export const getUserFixtures = (users: UserCreateDto[]) =>