Typescript error for custom type in schema

I can's figure out how to correctly type schema fields when creating custom types. For example, I have the following field definition in my schema.

  THUMBNAIL: {
    prop: 'thumbnail',
    required: true,
    type: (value: string) => {
      if (
        !value.toLowerCase().endsWith('.gif') &&
        !value.toLowerCase().endsWith('.jpg') &&
        !value.toLowerCase().endsWith('.jpeg') &&
        !value.toLowerCase().endsWith('.png') &&
        !value.toLowerCase().endsWith('.webp')
      ) {
        throw new Error(
          'Invalid thumbnail file type. Must be gif, jpg, jpeg, png, or webp'
        );
      }
      return value;
    },
  },

This works as expected and returns an error if the filetype is wrong. However, Typescript is throwing the following error:

No overload matches this call.
  Overload 1 of 3, '(input: File, options: ParseWithSchemaOptions<object>): Promise<ParsedObjectsResult<object>>', gave the following error.
    Type '{ AGE_DEMOGRAPHIC_FROM: { prop: string; type: NumberConstructor; }; AGE_DEMOGRAPHIC_TO: { prop: string; type: NumberConstructor; }; IMPRINT: { prop: string; type: StringConstructor; }; ... 9 more ...; TITLE: { ...; }; }' is not assignable to type 'Schema'.
      Property 'THUMBNAIL' is incompatible with index signature.
        Type '{ prop: string; required: boolean; type: (value: string) => string; }' is not assignable to type 'SchemaEntry'.
          Type '{ prop: string; required: boolean; type: (value: string) => string; }' is not assignable to type 'SchemaEntryRecursive'.
            Types of property 'type' are incompatible.
              Type '(value: string) => string' is not assignable to type 'Record<string, SchemaEntry>'.
                Index signature for type 'string' is missing in type '(value: string) => string'.
  Overload 2 of 3, '(input: File, options: ParseWithMapOptions): Promise<ParsedObjectsResult<object>>', gave the following error.
    Argument of type '{ schema: { AGE_DEMOGRAPHIC_FROM: { prop: string; type: NumberConstructor; }; AGE_DEMOGRAPHIC_TO: { prop: string; type: NumberConstructor; }; IMPRINT: { ...; }; ... 9 more ...; TITLE: { ...; }; }; }' is not assignable to parameter of type 'ParseWithMapOptions'.
      Object literal may only specify known properties, and 'schema' does not exist in type 'ParseWithMapOptions'.
  Overload 3 of 3, '(input: File, options?: ParseWithoutSchemaOptions | undefined): Promise<Row[]>', gave the following error.
    Argument of type '{ schema: { AGE_DEMOGRAPHIC_FROM: { prop: string; type: NumberConstructor; }; AGE_DEMOGRAPHIC_TO: { prop: string; type: NumberConstructor; }; IMPRINT: { ...; }; ... 9 more ...; TITLE: { ...; }; }; }' is not assignable to parameter of type 'ParseWithoutSchemaOptions'.
      Object literal may only specify known properties, and 'schema' does not exist in type 'ParseWithoutSchemaOptions'.ts(2769)
types.d.ts(67, 2): The expected type comes from property 'schema' which is declared here on type 'ParseWithSchemaOptions<object>'

Is there something else I should be doing or is this an issue?