diff --git a/README.md b/README.md
index 7c18593..4cdb255 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,7 @@ This library only does simple updates (e.g setting, modifying or deleting a valu
   * [Update an Array item](#update-array-item)
   * [Update a nested property using its current value](#update-nested-property-modify)
   * [Update a nested property on a nullable path](#update-nested-nullable-property)
+  * [Update a nested union property](#update-nested-union-property)
   * [Reuse a nested updater](#reuse-nested-updater)
 
 
@@ -246,6 +247,26 @@ deepUpdate<Person>({})
   .set('en')
 ```
 
+<a name="update-nested-union-property"></a>
+## Update a nested union property
+
+```ts
+import { deepUpdate } from 'immupdate'
+
+type A = { type: 'a', data: string }
+type B = { type: 'b', data: number }
+type Container = { aOrB: A | B }
+const isA = (u: A | B): u is A => u.type === 'a'
+
+const container = { aOrB: { type: 'a', data: 'aa' } }
+
+deepUpdate(container)
+  .at('aOrB')
+  .abortIfNot(isA)
+  .at('data')
+  .set('bb')
+```
+
 <a name="reuse-nested-updater"></a>
 ## Reuse a nested updater
 
diff --git a/immupdate.ts b/immupdate.ts
index 4ec2e27..8f795de 100644
--- a/immupdate.ts
+++ b/immupdate.ts
@@ -36,81 +36,101 @@ export const DELETE = {} as any as undefined
 export type ObjectLiteral = object & { reduceRight?: 'nope' }
 
 
-export interface AtUpdater<R, O> {
-  __T: [R, O] // Strengthen structural typing
+export interface AtUpdater<TARGET, CURRENT> {
+  __T: [TARGET, CURRENT] // Strengthen structural typing
 
   /**
    * Selects this Object key for update or further at() chaining
    */
-  at<K extends keyof O>(this: AtUpdater<R, ObjectLiteral>, key: K): Updater<R, O[K]>
+  at<K extends keyof CURRENT>(this: AtUpdater<TARGET, ObjectLiteral>, key: K): Updater<TARGET, CURRENT[K]>
 
   /**
    * Selects an Array index for update or further at() chaining
    */
-  at<A>(this: AtUpdater<R, A[]>, index: number): Updater<R, A | undefined>
+  at<A>(this: AtUpdater<TARGET, A[]>, index: number): Updater<TARGET, A | undefined>
 }
 
 // The at interface carrying a pre-bound value
-export interface BoundAtUpdater<R, O> {
-  __T: [R, O]
+export interface BoundAtUpdater<TARGET, CURRENT> {
+  __T: [TARGET, CURRENT]
 
   /**
    * Selects this Object key for update or further at() chaining
    */
-  at<K extends keyof O>(this: BoundAtUpdater<R, ObjectLiteral>, key: K): BoundUpdater<R, O[K]>
+  at<K extends keyof CURRENT>(this: BoundAtUpdater<TARGET, ObjectLiteral>, key: K): BoundUpdater<TARGET, CURRENT[K]>
 
   /**
    * Selects an Array index for update or further at() chaining
    */
-  at<A>(this: BoundAtUpdater<R, A[]>, index: number): BoundUpdater<R, A | undefined>
+  at<A>(this: BoundAtUpdater<TARGET, A[]>, index: number): BoundUpdater<TARGET, A | undefined>
 }
 
-export interface Updater<R, O> extends AtUpdater<R, O> {
-  __T: [R, O]
+export interface Updater<TARGET, CURRENT> extends AtUpdater<TARGET, CURRENT> {
+  __T: [TARGET, CURRENT]
 
   /**
    * Sets the value at the currently selected path.
    */
-  set(value: O): (target: R) => R
+  set(value: CURRENT): (target: TARGET) => TARGET
 
   /**
    * Modifies the value at the specified path. The current value is passed.
    */
-  modify(modifier: (value: O) => O): (target: R) => R
+  modify(modifier: (value: CURRENT) => CURRENT): (target: TARGET) => TARGET
 
   /**
    * Makes the previous nullable chain level 'safe' by using a default value
    */
-  withDefault<B, C extends B>(this: Updater<R, B | undefined>, defaultValue: C): Updater<R, B>
+  withDefault<B, C extends B>(this: Updater<TARGET, B | undefined>, defaultValue: C): Updater<TARGET, B>
 
   /**
    * Aborts the whole update operation if the previous chain level is null or undefined.
    */
-  abortIfUndef<B>(this: Updater<R, B | undefined>): Updater<R, B>
+  abortIfUndef<B>(this: Updater<TARGET, B | undefined>): Updater<TARGET, B>
+
+  /**
+   * Aborts the whole update operation if the previous chain level doesn't verify a type guard
+   */
+  abortIfNot<C extends CURRENT>(predicate: (value: CURRENT) => value is C): Updater<TARGET, C>
+
+  /**
+   * Aborts the whole update operation if the previous chain level doesn't verify a predicate
+   */
+  abortIfNot(predicate: (value: CURRENT) => boolean): Updater<TARGET, CURRENT>
 }
 
-export interface BoundUpdater<R, O> extends BoundAtUpdater<R, O> {
-  __T: [R, O]
+export interface BoundUpdater<TARGET, CURRENT> extends BoundAtUpdater<TARGET, CURRENT> {
+  __T: [TARGET, CURRENT]
 
   /**
    * Sets the value at the currently selected path.
    */
-  set(value: O): R
+  set(value: CURRENT): TARGET
 
   /**
    * Modifies the value at the specified path. The current value is passed.
    */
-  modify(modifier: (value: O) => O): R
+  modify(modifier: (value: CURRENT) => CURRENT): TARGET
 
   /**
    * Makes the previous nullable chain level 'safe' by using a default value
    */
-  withDefault<B, C extends B>(this: BoundUpdater<R, B | undefined>, defaultValue: C): BoundUpdater<R, B>
+  withDefault<B, C extends B>(this: BoundUpdater<TARGET, B | undefined>, defaultValue: C): BoundUpdater<TARGET, B>
 
   /**
    * Aborts the whole update operation if the previous chain level is null or undefined.
    */
-  abortIfUndef<B>(this: BoundUpdater<R, B | undefined>): BoundUpdater<R, B>
+  abortIfUndef<B>(this: BoundUpdater<TARGET, B | undefined>): BoundUpdater<TARGET, B>
+
+  /**
+   * Aborts the whole update operation if the previous chain level doesn't verify a type guard
+   */
+  abortIfNot<C extends CURRENT>(predicate: (value: CURRENT) => value is C): BoundUpdater<TARGET, C>
+
+  /**
+   * Aborts the whole update operation if the previous chain level doesn't verify a predicate
+   */
+  abortIfNot(predicate: (value: CURRENT) => boolean): BoundUpdater<TARGET, CURRENT>
 }
 
 
@@ -131,12 +151,13 @@ interface WithDefault {
   parent: any
 }
 
-interface AbortIfUndef {
-  type: 'abortIfUndef'
+interface AbortIfNot {
+  type: 'abortIfNot'
+  predicate: any
   parent: any
 }
 
-type UpdaterData = Root | At | WithDefault | AbortIfUndef
+type UpdaterData = Root | At | WithDefault | AbortIfNot
 
 
 
@@ -191,8 +212,12 @@ class _Updater {
     return new _Updater({ type: 'withDefault', parent: this, defaultValue: value })
   }
 
+  abortIfNot(predicate: any): any {
+    return new _Updater({ type: 'abortIfNot', parent: this, predicate })
+  }
+
   abortIfUndef(): any {
-    return new _Updater({ type: 'abortIfUndef', parent: this })
+    return this.abortIfNot((value: any) => value !== undefined)
   }
 
   findBoundTarget() {
@@ -227,13 +252,11 @@ class _Updater {
       return { host: newHost, field: newField }
     }
 
-    const value = previousHost[field]
-
-    if (this.data.type === 'abortIfUndef' && value === undefined) {
+    if (this.data.type === 'abortIfNot' && this.data.predicate(host) === false) {
       return { host, field, aborted: true }
     }
 
-    if (this.data.type === 'withDefault' && value === undefined) {
+    if (this.data.type === 'withDefault' && previousHost[field] === undefined) {
       const nextValue = this.data.defaultValue
       const newHost = isLast ? previousHost : nextValue
       previousHost[field] = nextValue
@@ -290,8 +313,8 @@ function clone(obj: any): any {
   return cloned
 }
 
-export function deepUpdate<O extends object>(target: O): BoundAtUpdater<O, O>
-export function deepUpdate<O extends object>(): AtUpdater<O, O>
+export function deepUpdate<TARGET extends object>(target: TARGET): BoundAtUpdater<TARGET, TARGET>
+export function deepUpdate<TARGET extends object>(): AtUpdater<TARGET, TARGET>
 export function deepUpdate(target?: any): any {
   return new _Updater({ type: 'root', boundTarget: target })
 }
\ No newline at end of file
diff --git a/package.json b/package.json
index 06f1eda..7fb27a9 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "immupdate",
-  "version": "1.1.6",
+  "version": "1.1.7",
   "description": "Immutable update for Objects and Arrays",
   "license": "MIT",
 
diff --git a/test/test.ts b/test/test.ts
index 4f9ea04..95779ea 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -658,6 +658,76 @@ describe('immupdate', () => {
       expect(updated4).toNotBe(obj2)
     })
 
+    it('can update an union member with an instance of this union', () => {
+
+      type A = { type: 'a', data: number }
+      type B = { type: 'b', data: string }
+      type U = A | B
+
+      type Obj = { u: U }
+
+      const obj: Obj = { u: { type: 'a', data: 10 } }
+
+      const updated = deepUpdate(obj)
+        .at('u')
+        .set({ type: 'b', data: '11' })
+
+      expect(updated).toEqual({
+        u: { type: 'b', data: '11' }
+      })
+    })
+
+    it('can abort if a simple condition is not met', () => {
+      type Obj = { a?: { version: number, data: string } }
+
+      const obj: Obj = { a: { version: 0, data: '001' } }
+
+      const updated = deepUpdate(obj)
+        .at('a')
+        .abortIfUndef()
+        .abortIfNot(a => a.version === 0)
+        .at('data')
+        .set('002')
+
+      expect(updated).toNotBe(obj)
+      expect(updated).toEqual({
+        a: { version: 0, data: '002' }
+      })
+
+      const obj2: Obj = { a: { version: 1, data: '001' } }
+      
+      const updated2 = deepUpdate(obj2)
+        .at('a')
+        .abortIfUndef()
+        .abortIfNot(a => a.version === 0)
+        .set({ version: 2, data: 'zzz' })
+
+      expect(updated2).toBe(obj2)
+    })
+
+    it('can abort if a type guard is not passed', () => {
+
+      type A = { type: 'a', data: number }
+      type B = { type: 'b', data: string }
+      type U = A | B
+
+      const isA = (u: U): u is A => u.type === 'a'
+
+      type Obj = { u: U }
+
+      const obj: Obj = { u: { type: 'a', data: 10 } }
+
+      const updated = deepUpdate(obj)
+        .at('u')
+        .abortIfNot(isA)
+        .at('data')
+        .set(20)
+
+      expect(updated).toEqual({
+        u: { type: 'a', data: 20 }
+      })
+    })
+
   })
 
 })
\ No newline at end of file