z.enum inside of z.tuple gives error "TypeError: Cannot convert undefined or null to object"

[gersomvg]

z.enum inside of z.tuple gives error: "TypeError: Cannot convert undefined or null to object"

Reproduction:

/**
 * Minimal reproduction of a bug in @asteasolutions/zod-to-openapi v8.4.3 with Zod v4.
 */
import { describe, it, expect } from "vitest";
import { z } from "zod";
import { extendZodWithOpenApi } from "@hono/zod-openapi";
import {
  OpenAPIRegistry,
  OpenApiGeneratorV3,
} from "@asteasolutions/zod-to-openapi";

extendZodWithOpenApi(z);

function generateDoc(schema: z.ZodType) {
  const registry = new OpenAPIRegistry();
  registry.registerPath({
    method: "post",
    path: "/test",
    request: {
      body: { content: { "application/json": { schema } } },
    },
    responses: { 200: { description: "ok" } },
  });
  return new OpenApiGeneratorV3(registry.definitions).generateDocument({
    openapi: "3.0.0",
    info: { title: "test", version: "1" },
  });
}

describe("zod-to-openapi bug: z.enum().nullable() inside z.tuple()", () => {
  it("works: z.enum() alone in a tuple", () => {
    const schema = z.tuple([z.enum(["a", "b"])]);
    expect(() => generateDoc(schema)).not.toThrow();
  });

  it("works: z.enum().nullable() outside a tuple", () => {
    const schema = z.enum(["a", "b"]).nullable();
    expect(() => generateDoc(schema)).not.toThrow();
  });

  it("works: z.string().nullable() inside a tuple", () => {
    const schema = z.tuple([z.string().nullable()]);
    expect(() => generateDoc(schema)).not.toThrow();
  });

  it("FAILS: z.enum().nullable() inside a tuple", () => {
    const schema = z.tuple([z.enum(["a", "b"]).nullable()]);
    // TypeError: Cannot convert undefined or null to object
    expect(() => generateDoc(schema)).not.toThrow();
  });

  it("FAILS: z.enum().nullable() inside an object inside a tuple", () => {
    const schema = z.tuple([
      z.object({ value: z.enum(["a", "b"]).nullable().optional() }),
    ]);
    // TypeError: Cannot convert undefined or null to object
    expect(() => generateDoc(schema)).not.toThrow();
  });
});