diff --git a/src/schema.ts b/src/schema.ts
index 533eca2..0a94227 100644
--- a/src/schema.ts
+++ b/src/schema.ts
@@ -6,7 +6,7 @@ import { regexpType } from './type/Regexp';
 import { streamType } from './type/Stream';
 
 export interface SchemaOptions {
-  include: IncludeOptions;
+  include: Readonly<IncludeOptions>;
 }
 
 /**
@@ -14,8 +14,8 @@ export interface SchemaOptions {
  *
  * @public
  */
-export function createSchema(options: SchemaOptions) {
-  const includeType = createInclude(options.include);
+export function createSchema(options: Readonly<SchemaOptions>) {
+  const {includeType, setSchema} = createInclude(options.include);
   const schema = DEFAULT_SCHEMA.extend([
     envType,
     includeType,
@@ -23,7 +23,7 @@ export function createSchema(options: SchemaOptions) {
     streamType,
   ]);
 
-  options.include.schema = schema;
+  setSchema(schema);
 
   return schema;
 }
diff --git a/src/type/Include.ts b/src/type/Include.ts
index 33fb21b..34e9379 100644
--- a/src/type/Include.ts
+++ b/src/type/Include.ts
@@ -1,5 +1,5 @@
-import { InvalidArgumentError, NotFoundError } from '@apextoaster/js-utils';
-import { load, Schema, Type as YamlType } from 'js-yaml';
+import { InvalidArgumentError, mustCoalesce, NotFoundError, Optional } from '@apextoaster/js-utils';
+import { DEFAULT_SCHEMA, load, Schema, Type as YamlType } from 'js-yaml';
 
 export type ReaderEncoding = 'ascii' | 'utf-8';
 export interface ReaderOptions {
@@ -14,15 +14,17 @@ export interface IncludeOptions {
   join: (...path: Array<string>) => string;
   read: IncludeReader;
   resolve: (path: string) => string;
-  schema: Schema;
+  schema: Optional<Schema>;
 }
 
 /**
  * Instantiate an includer with closure over the provided options.
  * @public
  */
-export function createInclude(options: IncludeOptions) {
-  return new YamlType('!include', {
+export function createInclude(options: Readonly<IncludeOptions>) {
+  const optionsCopy = {...options};
+
+  const includeType = new YamlType('!include', {
     kind: 'scalar',
     resolve(path: string) {
       try {
@@ -43,11 +45,21 @@ export function createInclude(options: IncludeOptions) {
         return load(options.read(abs, {
           encoding: 'utf-8',
         }), {
-          schema: options.schema,
+          schema: mustCoalesce(optionsCopy.schema, DEFAULT_SCHEMA),
         });
       } catch (err) {
         throw new InvalidArgumentError('error including file', err);
       }
     },
   });
+
+  // callback to avoid circular dependency (type must be created before schema)
+  function setSchema(schema: Schema) {
+    optionsCopy.schema = schema;
+  };
+
+  return {
+    includeType,
+    setSchema,
+  };
 }
diff --git a/test/type/TestInclude.ts b/test/type/TestInclude.ts
index 33478df..83dbd0a 100644
--- a/test/type/TestInclude.ts
+++ b/test/type/TestInclude.ts
@@ -17,12 +17,12 @@ const TEST_OPTIONS: IncludeOptions = {
 
 describe('include config type', async () => {
   it('should resolve existing files', async () => {
-    const includeType = createInclude(TEST_OPTIONS);
+    const {includeType} = createInclude(TEST_OPTIONS);
     expect(includeType.resolve(join(TEST_ROOT, 'include.yml'))).to.equal(true);
   });
 
   it('should throw when resolving missing files', async () => {
-    const includeType = createInclude({
+    const {includeType} = createInclude({
       ...TEST_OPTIONS,
       resolve: () => {
         throw new NotFoundError();
@@ -35,12 +35,12 @@ describe('include config type', async () => {
   });
 
   it('should construct data from file', async () => {
-    const includeType = createInclude(TEST_OPTIONS);
+    const {includeType} = createInclude(TEST_OPTIONS);
     expect(includeType.construct(join(TEST_ROOT, 'include.yml'))).to.equal('test');
   });
 
   it('should throw when constructing missing files', async () => {
-    const includeType = createInclude({
+    const {includeType} = createInclude({
       ...TEST_OPTIONS,
       read: () => {
         throw new InvalidArgumentError();