diff --git a/src/null-or.test.ts b/src/null-or.test.ts
new file mode 100644
index 0000000..d998d0a
--- /dev/null
+++ b/src/null-or.test.ts
@@ -0,0 +1,10 @@
+import * as $ from './index.js'
+
+test('simple', () => {
+  const union = $.or($.string, $.number)
+  const assert = $.nullOr(union)
+  expect(assert(null)).toEqual(null)
+  expect(assert(1)).toEqual(1)
+  expect(assert('foo')).toEqual('foo')
+  expect(() => assert(true)).toThrow('')
+})
diff --git a/src/or.test.ts b/src/or.test.ts
index 9c51dea..6c8ae80 100644
--- a/src/or.test.ts
+++ b/src/or.test.ts
@@ -1,6 +1,7 @@
 import * as $ from './index.js'
 
 test('or', () => {
-  const r = $.or($.number, $.string)(JSON.parse('"foo"'))
+  const assert = $.or($.number, $.string)
+  const r = assert(JSON.parse('"foo"'))
   expect(r).toEqual('foo')
 })
diff --git a/src/or.ts b/src/or.ts
index 6b39676..90d7141 100644
--- a/src/or.ts
+++ b/src/or.ts
@@ -1,12 +1,12 @@
-import type { Assert } from './prelude.js'
+import type { Assert, Asserted } from './prelude.js'
 import { inspect } from 'util'
 
-const or: <T extends Assert<unknown>[]>(...as: T) => T[number] =
-  (...as) =>
-    value => {
+const or =
+  <As extends Assert<unknown>[]>(...as: As) =>
+    (value: unknown) => {
       for (const a of as) {
         try {
-          return a(value)
+          return a(value) as Asserted<As[number]>
         } catch (err) {
           continue
         }
diff --git a/src/prelude.ts b/src/prelude.ts
index 76408d2..3d9cdfb 100644
--- a/src/prelude.ts
+++ b/src/prelude.ts
@@ -71,3 +71,6 @@ export type OptionalIfUndefinedOnly<T> = {
 
 export type OptionalIfUndefined<T> =
   Omit<T, keyof OptionalIfUndefinedOnly<T>> & OptionalIfUndefinedOnly<T>
+
+export type IntersectionOfUnion<T> =
+  (T extends unknown ? (_: T) => unknown : never) extends (_: infer R) => unknown ? R : never