diff --git a/src/Reflection/GenericParametersAcceptorResolver.php b/src/Reflection/GenericParametersAcceptorResolver.php
index 27d8cfc11c..e22958f0a2 100644
--- a/src/Reflection/GenericParametersAcceptorResolver.php
+++ b/src/Reflection/GenericParametersAcceptorResolver.php
@@ -59,6 +59,9 @@ public static function resolve(array $argTypes, ParametersAcceptor $parametersAc
 		foreach ($parameters as $param) {
 			if (isset($namedArgTypes[$param->getName()])) {
 				$argType = $namedArgTypes[$param->getName()];
+				if ($argType instanceof ErrorType && $param->getDefaultValue() !== null) {
+					$argType = $param->getDefaultValue();
+				}
 			} elseif ($param->getDefaultValue() !== null) {
 				$argType = $param->getDefaultValue();
 			} else {
diff --git a/tests/PHPStan/Analyser/nsrt/bug-12687.php b/tests/PHPStan/Analyser/nsrt/bug-12687.php
new file mode 100644
index 0000000000..5bb9e142e9
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-12687.php
@@ -0,0 +1,42 @@
+<?php declare(strict_types = 1);
+
+namespace Bug12687;
+
+use function PHPStan\Testing\assertType;
+
+class A
+{
+}
+
+interface I
+{
+	function method(): void;
+}
+
+class Testing
+{
+	/**
+	 * @param A|null $arg
+	 * @param-out ($arg is null ? A&I : A) $arg
+	 */
+	public static function testMethod(?A &$arg = null): void
+	{
+		if ($arg === null) {
+			$arg = new A();
+		}
+	}
+
+	public function doTest(): void
+	{
+		$a = new A();
+		Testing::testMethod($a);
+		assertType('Bug12687\A', $a);
+
+		$b = null;
+		Testing::testMethod($b);
+		assertType('Bug12687\A&Bug12687\I', $b);
+
+		Testing::testMethod($c);
+		assertType('Bug12687\A&Bug12687\I', $c);
+	}
+}