diff --git a/src/Reflection/Type/UnionTypeMethodReflection.php b/src/Reflection/Type/UnionTypeMethodReflection.php
index b330b6fdad..072d950882 100644
--- a/src/Reflection/Type/UnionTypeMethodReflection.php
+++ b/src/Reflection/Type/UnionTypeMethodReflection.php
@@ -177,7 +177,13 @@ public function getDocComment(): ?string
 
 	public function getAsserts(): Assertions
 	{
-		return Assertions::createEmpty();
+		$assertions = Assertions::createEmpty();
+
+		foreach ($this->methods as $method) {
+			$assertions = $assertions->intersectWith($method->getAsserts());
+		}
+
+		return $assertions;
 	}
 
 	public function acceptsNamedArguments(): TrinaryLogic
diff --git a/tests/PHPStan/Analyser/nsrt/bug-11441.php b/tests/PHPStan/Analyser/nsrt/bug-11441.php
new file mode 100644
index 0000000000..d1c497ec17
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-11441.php
@@ -0,0 +1,39 @@
+<?php declare(strict_types=1);
+
+namespace Bug11441;
+
+use function PHPStan\Testing\assertType;
+
+class Foo {
+    public function getParam(): ?string {
+        return 'foo';
+    }
+
+    /** @phpstan-assert !null $this->getParam() */
+    public function checkNotNull(): void {
+        if ($this->getParam() === null) {
+            throw new \Exception();
+        }
+    }
+}
+
+class Bar {
+    public function getParam(): ?int {
+        return 1;
+    }
+
+    /** @phpstan-assert !null $this->getParam() */
+    public function checkNotNull(): void {
+        if ($this->getParam() === null) {
+            throw new \Exception();
+        }
+    }
+}
+
+function test(Foo|Bar $obj): void {
+    assertType('int|string|null', $obj->getParam());
+
+    $obj->checkNotNull();
+
+    assertType('int|string', $obj->getParam());
+}
diff --git a/tests/PHPStan/Analyser/nsrt/bug-13358.php b/tests/PHPStan/Analyser/nsrt/bug-13358.php
new file mode 100644
index 0000000000..7c8af419e3
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-13358.php
@@ -0,0 +1,40 @@
+<?php declare(strict_types=1);
+
+namespace Bug13358;
+
+use function PHPStan\Testing\assertType;
+
+/**
+ * @phpstan-sealed SystemActor|AnonymousVisitorActor
+ */
+abstract class Actor
+{
+    /**
+     * @phpstan-assert-if-true SystemActor $this
+     */
+    public function isSystem(): bool
+    {
+        return $this instanceof SystemActor;
+    }
+
+    /**
+     * @phpstan-assert-if-true AnonymousVisitorActor $this
+     */
+    public function isAnonymousVisitor(): bool
+    {
+        return $this instanceof AnonymousVisitorActor;
+    }
+}
+
+class SystemActor extends Actor {}
+class AnonymousVisitorActor extends Actor {}
+
+function test(AnonymousVisitorActor|SystemActor $actor): void {
+    assertType('Bug13358\AnonymousVisitorActor|Bug13358\SystemActor', $actor);
+
+    if ($actor->isSystem()) {
+        assertType('Bug13358\SystemActor', $actor);
+    } else {
+        assertType('Bug13358\AnonymousVisitorActor', $actor);
+    }
+}