How to create an optional many:1 relationships from one table to itself in a Postgres database using Prisma?

Question

I'm using Prisma with a Postgres database. I have the following model in my Prisma schema, the part of the model in question is the optional oldVersionId:

model Block {
  id                String              @id @default(uuid())
  createdAt         DateTime            @default(now())
  updatedAt         DateTime            @updatedAt
  actingUser        User                @relation(fields: [userId], references: [id])
  userId            String
  accessType        AccessType          @default(PRIVATE)
  blockType         BlockType           @default(IMPERATIVE)
  blockState        BlockState          @default(NOT_STARTED)
  humanText         String
  parents           HierarchyRelation[] @relation("parents")
  children          HierarchyRelation[] @relation("children")
  workspace         String
  workspaceOutgoing ReferenceRelation[] @relation("outgoing")
  workspaceIncoming ReferenceRelation[] @relation("incoming")
  oldVersion        Block?              @relation("lineage", fields: [oldVersionId], references: [id])
  oldVersionId      String?
  newVersions       Block[]             @relation("lineage")
}

I want oldVersionId to be null in case the object is the original version. However, when I query the database with prisma.block.findMany(), I get the following error:

Invalid `prisma.block.findMany()` invocation: Attempted to serialize scalar 'null' with incompatible type 'String' for field oldVersionId.

Why is the null type not able to be serialized, and how else could I set up an optional many:1 relationship?