diff --git a/src/UnionParameter.php b/src/UnionParameter.php
index 8201b38..0068c4e 100644
--- a/src/UnionParameter.php
+++ b/src/UnionParameter.php
@@ -20,6 +20,9 @@
 use Chevere\Parameter\Traits\ArrayParameterTrait;
 use Chevere\Parameter\Traits\ParameterAssertArrayTypeTrait;
 use Chevere\Parameter\Traits\ParameterTrait;
+use Throwable;
+use TypeError;
+use function Chevere\Message\message;
 
 final class UnionParameter implements UnionParameterInterface
 {
@@ -42,7 +45,28 @@ final public function __construct(
 
     public function __invoke(mixed $value): mixed
     {
-        return assertUnion($this, $value);
+        $types = [];
+        $errors = [];
+        foreach ($this->parameters() as $item) {
+            try {
+                assertNamedArgument('', $item, $value);
+
+                return $value;
+            } catch (Throwable $e) {
+                $types[] = $item::class;
+                $errors[] = $e->getMessage();
+            }
+        }
+        $types = implode('|', $types);
+        $errors = implode('; ', $errors);
+
+        throw new TypeError(
+            (string) message(
+                "Argument provided doesn't match the union type `%types%`. Error(s): %errors%",
+                types: $types,
+                errors: $errors,
+            )
+        );
     }
 
     public function withAdded(ParameterInterface ...$parameter): static
diff --git a/src/functions.php b/src/functions.php
index ae45719..3f07c49 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -35,7 +35,6 @@
 use ReflectionMethod;
 use ReflectionParameter;
 use Throwable;
-use TypeError;
 use function Chevere\Message\message;
 
 function cast(mixed $argument): CastInterface
@@ -92,29 +91,6 @@ function arguments(
     return new Arguments($parameters, $arguments);
 }
 
-function assertUnion(
-    UnionParameterInterface $parameter,
-    mixed $argument,
-): mixed {
-    $types = [];
-    foreach ($parameter->parameters() as $item) {
-        try {
-            assertNamedArgument('', $item, $argument);
-
-            return $argument;
-        } catch (Throwable $e) {
-            $types[] = $item::class;
-        }
-    }
-
-    throw new TypeError(
-        (string) message(
-            "Argument provided doesn't match the union type `%type%`",
-            type: implode('|', $types)
-        )
-    );
-}
-
 function assertNamedArgument(
     string $name,
     ParameterInterface $parameter,
diff --git a/tests/FunctionsTest.php b/tests/FunctionsTest.php
index cd8c8ac..f8486bf 100644
--- a/tests/FunctionsTest.php
+++ b/tests/FunctionsTest.php
@@ -24,7 +24,6 @@
 use function Chevere\Parameter\arrayp;
 use function Chevere\Parameter\assertArray;
 use function Chevere\Parameter\assertNamedArgument;
-use function Chevere\Parameter\assertUnion;
 use function Chevere\Parameter\bool;
 use function Chevere\Parameter\float;
 use function Chevere\Parameter\generic;
@@ -37,7 +36,6 @@
 use function Chevere\Parameter\string;
 use function Chevere\Parameter\takeFrom;
 use function Chevere\Parameter\takeKeys;
-use function Chevere\Parameter\union;
 
 final class FunctionsTest extends TestCase
 {
@@ -211,18 +209,6 @@ public function testFunctionGenericParameter(): void
         $this->assertSame('foo', $parameter->description());
     }
 
-    public function testFunctionUnionParameter(): void
-    {
-        $parameter = union(
-            int(),
-            string(),
-        );
-        assertUnion($parameter, 'foo');
-        assertUnion($parameter, 123);
-        $this->expectException(TypeError::class);
-        assertUnion($parameter, []);
-    }
-
     public function testAssertArrayExtraArguments(): void
     {
         $parameter = arrayp(
diff --git a/tests/FunctionsUnionTest.php b/tests/FunctionsUnionTest.php
deleted file mode 100644
index 93eda36..0000000
--- a/tests/FunctionsUnionTest.php
+++ /dev/null
@@ -1,105 +0,0 @@
-<?php
-
-/*
- * This file is part of Chevere.
- *
- * (c) Rodolfo Berrios <rodolfo@chevere.org>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-declare(strict_types=1);
-
-namespace Chevere\Tests;
-
-use InvalidArgumentException;
-use PHPUnit\Framework\TestCase;
-use TypeError;
-use function Chevere\Parameter\arrayp;
-use function Chevere\Parameter\assertGeneric;
-use function Chevere\Parameter\assertUnion;
-use function Chevere\Parameter\generic;
-use function Chevere\Parameter\int;
-use function Chevere\Parameter\null;
-use function Chevere\Parameter\union;
-
-final class FunctionsUnionTest extends TestCase
-{
-    public function testUnionArrayFixed(): void
-    {
-        $array = arrayp(a: int());
-        $union = union(arrayp(), $array);
-        $argument = [
-            'a' => 1,
-        ];
-        assertUnion($union, []);
-        assertUnion($union, $argument);
-        $union = union($array, null());
-        assertUnion($union, $argument);
-        $this->expectException(TypeError::class);
-        assertUnion($union, []);
-    }
-
-    public function testUnionArrayGeneric(): void
-    {
-        $array = arrayp(a: int());
-        $generic = generic($array);
-        $union = union(arrayp(), $generic);
-        $argument = [
-            [
-                'a' => 1,
-            ],
-            [
-                'a' => 2,
-            ],
-        ];
-        assertUnion($union, []);
-        assertUnion($union, $argument);
-        $union = union($generic, null());
-        assertUnion($union, $argument);
-        $this->expectException(TypeError::class);
-        assertUnion($union, []);
-    }
-
-    public function testUnionGenericEmptyArray(): void
-    {
-        $array = arrayp(a: int());
-        $union = union(arrayp(), $array);
-        $generic = generic($union);
-        $argument = [
-            [
-                'a' => 1,
-            ],
-            [
-                'a' => 2,
-            ],
-        ];
-        assertGeneric($generic, $argument);
-        assertGeneric($generic, [[]]);
-        $generic = generic(union($array, null()));
-        assertGeneric($generic, $argument);
-        $this->expectException(InvalidArgumentException::class);
-        assertGeneric($generic, [[]]);
-    }
-
-    public function testUnionGeneric(): void
-    {
-        $generic = generic(int());
-        $union = union(arrayp(), $generic);
-        assertUnion($union, []);
-        assertUnion($union, [1, 2, 3]);
-        $this->expectException(TypeError::class);
-        assertUnion($union, [[]]);
-    }
-
-    public function testUnionGenericArray(): void
-    {
-        $generic = generic(arrayp());
-        $union = union(arrayp(), $generic);
-        assertUnion($union, []);
-        assertUnion($union, [[]]);
-        $this->expectException(TypeError::class);
-        assertUnion($union, [[[]]]);
-    }
-}