Composite Schema. Error: "Cannot read property of undefined"

Validation nesting schemas throws an error if

Composite Schema
---
```
const user = schema({
email: String
});

const querySchema = schema({
user,
startDate: Date,
endDate: Date
});
```

Validation
---
```
await querySchema.validate({
startDate: (new Date())
});
```

the object I'm passing to `validate` method doesn't have key `user`. And since `user` not mandatory I expect validation to pass, but instead it throws `Error: Cannot read property 'email' of undefined`

Is it expecting behaviour?