why am I getting this error when trying to update 1 user and using null in where

    await db.user.update({
      where: {
        id: user.id,
        username: null,
      },
      data: {
        username: result.data.username,
      },
    });

// This is your Prisma schema file,
// learn more about it in the docs: https://pris.ly/d/prisma-schema

// Looking for ways to speed up your queries, or scale easily with your serverless or edge functions?
// Try Prisma Accelerate: https://pris.ly/cli/accelerate-init

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["postgresqlExtensions", "strictUndefinedChecks"]
}

datasource db {
  provider   = "postgresql"
  url        = env("DATABASE_URL")
  extensions = [citext(schema: "pg_catalog")]
}

model User {
  id        String  @id @default(cuid())
  username  String? @unique @db.Citext
  githubId  String? @unique
  email     String  @db.VarChar(200)
  avatarUrl String?
  // avatarSource
  Post      Post[]

  @@map("users")
}

model Post {
  id     String @id @default(cuid())
  title  String @db.VarChar(20)
  userId String

  user User @relation(fields: [userId], references: [id], onDelete: Cascade)

  @@map("posts")
}