diff --git a/src/Analyser/TypeSpecifier.php b/src/Analyser/TypeSpecifier.php
index e946229f7d..af9497da65 100644
--- a/src/Analyser/TypeSpecifier.php
+++ b/src/Analyser/TypeSpecifier.php
@@ -21,6 +21,7 @@
 use PhpParser\Node\Name;
 use PHPStan\DependencyInjection\AutowiredService;
 use PHPStan\Node\Expr\AlwaysRememberedExpr;
+use PHPStan\Node\Expr\TypeExpr;
 use PHPStan\Node\IssetExpr;
 use PHPStan\Node\Printer\ExprPrinter;
 use PHPStan\Php\PhpVersion;
@@ -1451,6 +1452,14 @@ private function specifyTypesFromAsserts(TypeSpecifierContext $context, Expr\Cal
 
 			$argsMap[$paramName][] = $arg->value;
 		}
+		foreach ($parameters as $parameter) {
+			$name = $parameter->getName();
+			$defaultValue = $parameter->getDefaultValue();
+			if (isset($argsMap[$name]) || $defaultValue === null) {
+				continue;
+			}
+			$argsMap[$name][] = new TypeExpr($defaultValue);
+		}
 
 		if ($call instanceof MethodCall) {
 			$argsMap['this'] = [$call->var];
diff --git a/tests/PHPStan/Analyser/nsrt/bug-9435.php b/tests/PHPStan/Analyser/nsrt/bug-9435.php
new file mode 100644
index 0000000000..a3196862b1
--- /dev/null
+++ b/tests/PHPStan/Analyser/nsrt/bug-9435.php
@@ -0,0 +1,177 @@
+<?php // lint >= 8.0
+
+declare(strict_types=1);
+
+namespace Bug9435;
+
+use function PHPStan\Testing\assertType;
+
+function x(): mixed
+{
+	return null;
+}
+
+/** @phpstan-assert ($allow_null is true ? string|null : string) $input */
+function trueCheck(mixed $input, bool $allow_null = false): void
+{
+}
+
+$a = x();
+trueCheck($a);
+assertType('string', $a); // incorrect: should be string but is string|null
+
+$a = x();
+trueCheck($a, false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+trueCheck($a, allow_null: false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+trueCheck(allow_null: false, input: $a);
+assertType('string', $a); // correct (string)
+
+$a = x();
+trueCheck($a, true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+trueCheck($a, allow_null: true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+trueCheck(allow_null: true, input: $a);
+assertType('string|null', $a); // correct (string|null)
+
+/** @phpstan-assert ($allow_null is false ? string : string|null) $input */
+function falseCheck(mixed $input, bool $allow_null = false): void
+{
+}
+
+$a = x();
+falseCheck($a);
+assertType('string', $a); // incorrect: should be string but is string|null
+
+$a = x();
+falseCheck($a, false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+falseCheck($a, allow_null: false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+falseCheck(allow_null: false, input: $a);
+assertType('string', $a); // correct (string|null)
+
+$a = x();
+falseCheck($a, true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+falseCheck($a, allow_null: true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+falseCheck(allow_null: true, input: $a);
+assertType('string|null', $a); // correct (string|null)
+
+/** @phpstan-assert ($allow_null is not true ? string : string|null) $input */
+function notTrueCheck(mixed $input, bool $allow_null = false): void
+{
+}
+
+$a = x();
+notTrueCheck($a);
+assertType('string', $a); // incorrect: should be string but is string|null
+
+$a = x();
+notTrueCheck($a, false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+notTrueCheck($a, allow_null: false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+notTrueCheck(allow_null: false, input: $a);
+assertType('string', $a); // correct (string|null)
+
+$a = x();
+notTrueCheck($a, true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+notTrueCheck($a, allow_null: true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+notTrueCheck(allow_null: true, input: $a);
+assertType('string|null', $a); // correct (string|null)
+
+/** @phpstan-assert ($allow_null is not false ? string|null : string) $input */
+function notFalseCheck(mixed $input, bool $allow_null = false): void
+{
+}
+
+$a = x();
+notFalseCheck($a);
+assertType('string', $a); // incorrect: should be string but is string|null
+
+$a = x();
+notFalseCheck($a, false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+notFalseCheck($a, allow_null: false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+notFalseCheck(allow_null: false, input: $a);
+assertType('string', $a); // correct (string|null)
+
+$a = x();
+notFalseCheck($a, true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+notFalseCheck($a, allow_null: true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+notFalseCheck(allow_null: true, input: $a);
+assertType('string|null', $a); // correct (string|null)
+
+/** @phpstan-assert ($allow_null is false ? string : string|null) $input */
+function checkWithVariadics(mixed $input, bool $allow_null = false, ...$more): void
+{
+}
+
+$a = x();
+checkWithVariadics($a);
+assertType('string', $a); // incorrect: should be string but is string|null
+
+$a = x();
+checkWithVariadics($a, false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+checkWithVariadics($a, allow_null: false);
+assertType('string', $a); // correct (string)
+
+$a = x();
+checkWithVariadics(allow_null: false, input: $a);
+assertType('string', $a); // correct (string)
+
+$a = x();
+checkWithVariadics($a, true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+checkWithVariadics($a, allow_null: true);
+assertType('string|null', $a); // correct (string|null)
+
+$a = x();
+checkWithVariadics(allow_null: true, input: $a);
+assertType('string|null', $a); // correct (string|null)