Validation failed: Cannot read property 'caster' of undefined

[lcrosetto]

Do you want to request a feature or report a bug?

Bug

What is the current behavior?

Calling model.validate() on a document field with an array of strings throws an error.

Error: Validation failed: docs.0: Cannot read property 'caster' of undefined
    at ValidationError.inspect (node_modules/mongoose/lib/error/validation.js:48:26)
    at internal/per_context/primordials.js:23:32
    at formatValue (internal/util/inspect.js:774:19)
    at inspect (internal/util/inspect.js:336:10)
    at formatWithOptionsInternal (internal/util/inspect.js:2006:40)
    at formatWithOptions (internal/util/inspect.js:1888:10)
    at console.value (internal/console/constructor.js:320:14)
    at console.warn (internal/console/constructor.js:353:61)
    at validate (bugtest.js:15:13)
    at processTicksAndRejections (internal/process/task_queues.js:93:5) {
  errors: {
    'docs.0': TypeError: Cannot read property 'caster' of undefined
        at Proxy._cast (node_modules/mongoose/lib/types/array/methods/index.js:245:36)
        at Object.set (node_modules/mongoose/lib/types/array/index.js:72:50)
        at node_modules/mongoose/lib/model.js:4261:42
        at node_modules/mongoose/lib/helpers/promiseOrCallback.js:32:5
        at new Promise (<anonymous>)
        at promiseOrCallback (node_modules/mongoose/lib/helpers/promiseOrCallback.js:31:10)
        at Mongoose._promiseOrCallback (node_modules/mongoose/lib/index.js:1151:10)
        at Function.validate (node_modules/mongoose/lib/model.js:4208:23)
        at validate (bugtest.js:13:28)
        at Object.<anonymous> (bugtest.js:19:1)
  },
  _message: 'Validation failed'
}

If the current behavior is a bug, please provide the steps to reproduce.

const { Schema, model } = require('mongoose');

const testSchema = new Schema({
  docs: [String],
});

const Test = model('Test', testSchema);

const test = { docs: [ '6132655f2cdb9d94eaebc09b' ] };

async function validate() {
  try {
    const res = await Test.validate(test);
  } catch (e) {
    console.error(e);
  }
}

validate();

What is the expected behavior?

Validate should succeed. It functions without error in v5.13.8.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Node: 14.15.1
Mongoose: 6.0.4
MongoDB: 4.4.8