z.coerce.date() rendered as nullable

[tmokmss]

Hi, I have a question about the conversion behavior of z.coerce.date.

With the below code:

import { OpenApiGeneratorV31, extendZodWithOpenApi } from "@asteasolutions/zod-to-openapi";
import { z } from 'zod';
extendZodWithOpenApi(z);

const UserSchema = z
  .object({
    createdAt: z.coerce.date(),
  })
  .openapi('User');

const generator = new OpenApiGeneratorV31([UserSchema]);
const schema = generator.generateComponents();
console.log(JSON.stringify(schema, undefined, 2));

we get the following output:

{
  "components": {
    "schemas": {
      "User": {
        "type": "object",
        "properties": {
          "createdAt": {
            "type": [
              "string",
              "null"
            ]
          }
        },
        "required": [
          "createdAt"
        ]
      }
    },
    "parameters": {}
  }
}

The problem for me is that createdAt is rendered as nullable, where I want it to render as just string. Is there any workaround? I guess it contains null because z.coerce.date().parse(null) is valid, but not sure if it makes sense when it comes to openapi response schema.

Context: I use zod-prisma-types to generate zod schema from prisma schema, and it generates z.coerce.string() from a DateTime column as below:

// schema.prisma
model User {
  createdAt DateTime
}

// the generated zod type
export const User = z.object({
  createdAt: z.coerce.date(),
})

And when I want to directly use the schema as response type, the createdAt field unexpectedly becomes nullable.

Thanks!