diff --git a/src/struct.ts b/src/struct.ts
index 9a5c37ec..e3a94f28 100644
--- a/src/struct.ts
+++ b/src/struct.ts
@@ -86,7 +86,8 @@ export class Struct<T = unknown, S = unknown> {
 
   /**
    * Mask a value, coercing and validating it, but returning only the subset of
-   * properties defined by the struct's schema.
+   * properties defined by the struct's schema. Masking applies recursively to
+   * props of `object` structs only.
    */
 
   mask(value: unknown, message?: string): T {
@@ -97,15 +98,17 @@ export class Struct<T = unknown, S = unknown> {
    * Validate a value with the struct's validation logic, returning a tuple
    * representing the result.
    *
-   * You may optionally pass `true` for the `withCoercion` argument to coerce
+   * You may optionally pass `true` for the `coerce` argument to coerce
    * the value before attempting to validate it. If you do, the result will
-   * contain the coerced result when successful.
+   * contain the coerced result when successful. Also, `mask` will turn on
+   * masking of the unknown `object` props recursively if passed.
    */
 
   validate(
     value: unknown,
     options: {
       coerce?: boolean
+      mask?: boolean
       message?: string
     } = {}
   ): [StructError, undefined] | [undefined, T] {
@@ -209,12 +212,16 @@ export function validate<T, S>(
 
 /**
  * A `Context` contains information about the current location of the
- * validation inside the initial input value.
+ * validation inside the initial input value. It also carries `mask`
+ * since it's a run-time flag determining how the validation was invoked
+ * (via `mask()` or via `validate()`), plus it applies recursively
+ * to all of the nested structs.
  */
 
 export type Context = {
   branch: Array<any>
   path: Array<any>
+  mask?: boolean
 }
 
 /**
diff --git a/src/structs/types.ts b/src/structs/types.ts
index 19493104..372aa5b1 100644
--- a/src/structs/types.ts
+++ b/src/structs/types.ts
@@ -319,8 +319,25 @@ export function object<S extends ObjectSchema>(schema?: S): any {
         isObject(value) || `Expected an object, but received: ${print(value)}`
       )
     },
-    coercer(value) {
-      return isObject(value) ? { ...value } : value
+    coercer(value, ctx) {
+      if (!isObject(value) || Array.isArray(value)) {
+        return value
+      }
+
+      const coerced = { ...value }
+
+      // The `object` struct has special behaviour enabled by the mask flag.
+      // When masking, properties that are not in the schema are deleted from
+      // the coerced object instead of eventually failing validaiton.
+      if (ctx.mask && schema) {
+        for (const key in coerced) {
+          if (schema[key] === undefined) {
+            delete coerced[key]
+          }
+        }
+      }
+
+      return coerced
     },
   })
 }
@@ -499,9 +516,12 @@ export function union<A extends AnyStruct, B extends AnyStruct[]>(
   return new Struct({
     type: 'union',
     schema: null,
-    coercer(value) {
+    coercer(value, ctx) {
       for (const S of Structs) {
-        const [error, coerced] = S.validate(value, { coerce: true })
+        const [error, coerced] = S.validate(value, {
+          coerce: true,
+          mask: ctx.mask,
+        })
         if (!error) {
           return coerced
         }
diff --git a/src/utils.ts b/src/utils.ts
index db69aea1..aa80983e 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -131,24 +131,10 @@ export function* run<T, S>(
   } = {}
 ): IterableIterator<[Failure, undefined] | [undefined, T]> {
   const { path = [], branch = [value], coerce = false, mask = false } = options
-  const ctx: Context = { path, branch }
+  const ctx: Context = { path, branch, mask }
 
   if (coerce) {
     value = struct.coercer(value, ctx)
-
-    if (
-      mask &&
-      struct.type !== 'type' &&
-      isObject(struct.schema) &&
-      isObject(value) &&
-      !Array.isArray(value)
-    ) {
-      for (const key in value) {
-        if (struct.schema[key] === undefined) {
-          delete value[key]
-        }
-      }
-    }
   }
 
   let status: 'valid' | 'not_refined' | 'not_valid' = 'valid'
diff --git a/test/api/mask.test.ts b/test/api/mask.test.ts
index 754fac30..fee3d99f 100644
--- a/test/api/mask.test.ts
+++ b/test/api/mask.test.ts
@@ -7,6 +7,7 @@ import {
   StructError,
   array,
   type,
+  union,
 } from '../../src'
 
 describe('mask', () => {
@@ -44,19 +45,36 @@ describe('mask', () => {
   it('masking of a nested type', () => {
     const S = object({
       id: string(),
-      sub: array(type({ prop: string() })),
+      sub: array(
+        type({ prop: string(), defaultedProp: defaulted(string(), '42') })
+      ),
+      union: array(union([object({ prop: string() }), type({ k: string() })])),
     })
     const value = {
       id: '1',
       unknown: true,
       sub: [{ prop: '2', unknown: true }],
+      union: [
+        { prop: '3', unknown: true },
+        { k: '4', unknown: true },
+      ],
     }
     expect(mask(value, S)).toStrictEqual({
       id: '1',
-      sub: [{ prop: '2', unknown: true }],
+      sub: [{ prop: '2', unknown: true, defaultedProp: '42' }],
+      union: [{ prop: '3' }, { k: '4', unknown: true }],
     })
   })
 
+  it('masking succeeds for objects with extra props in union', () => {
+    const S = union([
+      object({ a: string(), defaultedProp: defaulted(string(), '42') }),
+      object({ b: string() }),
+    ])
+    const value = { a: '1', extraProp: 42 }
+    expect(mask(value, S)).toStrictEqual({ a: '1', defaultedProp: '42' })
+  })
+
   it('masking of a top level type and nested object', () => {
     const S = type({
       id: string(),