Using `define` with `strictNullChecks`

I am having trouble using a custom type I have defined. Please see below my use case:

/**
 * The interface I would like to validate.
 */
export interface FsAppAccess {
    code?: string;
    organisation: admin.firestore.DocumentReference;
    organisationHierarchy: admin.firestore.DocumentReference[];
}

/**
 * My custom type for admin.firestore.DocumentReference.
 */
export const organisationDocumentReference = define<admin.firestore.DocumentReference>('organisations-document-reference', (value: admin.firestore.DocumentReference) => {
  return is(value.path, pattern(string(), /^organisations\/[^\/]+$/));
});

/**
 * The complete validation struct.
 */
export const fsAppAccessStruct: Describe<FsAppAccess> = object({
    code: optional(string()),
    organisation: organisationDocumentReference,
    organisationHierarchy: length(array(organisationDocumentReference), 1)
});

admin.firestore.DocumentReference is a class defined in a third party library Firebase. I’m only interested that the object has a specific path, rather than fully validating the whole object, which is unnecessary for my use case. I thought I could accomplish this using the define utility, which had worked perfectly at the time I implemented.

I have since modified my tsconfig to use strictNullChecks, which is now causing a compiler error:

Types of property 'organisation' are incompatible.       
Type 'Struct<DocumentReference<DocumentData>, null>' is not assignable to type 'Describe<DocumentReference<DocumentData>>'

If I modify the line:

organisation: organisationDocumentReference;

to

organisation: organisationDocumentReference | null;

then the problem goes away, but that is definitely not what I want. The property should not be allowed to be null.

Am I holding it wrong?

[ianstormtaylor]

@Poliziano can you create a minimal reproduction of this?

[j]

I just ran into this issue as well.