diff --git a/src/index.ts b/src/index.ts
index e174bdd..fdf94d6 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -256,13 +256,12 @@ const mkMatchXW =
   <C extends B[keyof B]>(x: A): C => {
     const tag = x[tagKey] as Tag<A>
 
-    const g = fs[tag]
     // eslint-disable-next-line functional/no-conditional-statement, @typescript-eslint/no-unsafe-return
-    if (g !== undefined) return g as C
+    if (Object.prototype.hasOwnProperty.call(fs, tag)) return fs[tag] as C
 
-    const h = (fs as CasesXWildcard<A, B>)[_]
     // eslint-disable-next-line functional/no-conditional-statement, @typescript-eslint/no-unsafe-return
-    if (h !== undefined) return h as C
+    if (Object.prototype.hasOwnProperty.call(fs, _))
+      return (fs as CasesXWildcard<A, B>)[_] as C
 
     // eslint-disable-next-line functional/no-throw-statement
     throw new Error(`Failed to pattern match against tag "${tag}".`)
diff --git a/test/unit/index.ts b/test/unit/index.ts
index 5a531a8..2e6182a 100644
--- a/test/unit/index.ts
+++ b/test/unit/index.ts
@@ -84,6 +84,22 @@ describe("index", () => {
         expect(f(Weather.mk.Sun)).toBe("didn't rain")
       })
 
+      it("matchX allow undefined value", () => {
+        type T = Member<"A"> | Member<"B"> | Member<"toString">
+        const T = create<T>()
+
+        const f = T.matchX({
+          A: undefined,
+          [_]: undefined,
+        })
+
+        expect(f(T.mk.A)).toBe(undefined)
+        expect(f(T.mk.B)).toBe(undefined)
+        // Check that `toString` does not match an object prototype function
+        // This check would fail when the `in` operator is used instead of `hasOwnProperty`.
+        expect(f(T.mk.toString)).toBe(undefined)
+      })
+
       it("matchXW", () => {
         const f = Weather.matchXW({
           Rain: "rained",
@@ -98,6 +114,22 @@ describe("index", () => {
 
         expect(f(Weather.mk.Sun)).toBeNull()
       })
+
+      it("matchXW allow undefined value", () => {
+        type T = Member<"A"> | Member<"B"> | Member<"toString">
+        const T = create<T>()
+
+        const f = T.matchXW({
+          A: undefined,
+          [_]: undefined,
+        })
+
+        expect(f(T.mk.A)).toBe(undefined)
+        expect(f(T.mk.B)).toBe(undefined)
+        // Check that `toString` does not match an object prototype function.
+        // This check would fail when the `in` operator is used instead of `hasOwnProperty`.
+        expect(f(T.mk.toString)).toBe(undefined)
+      })
     })
   })