diff --git a/spec/Prophecy/Doubler/ClassPatch/DisableConstructorPatchSpec.php b/spec/Prophecy/Doubler/ClassPatch/DisableConstructorPatchSpec.php
index 2ffb5cc8..0479aa29 100644
--- a/spec/Prophecy/Doubler/ClassPatch/DisableConstructorPatchSpec.php
+++ b/spec/Prophecy/Doubler/ClassPatch/DisableConstructorPatchSpec.php
@@ -5,6 +5,7 @@
 use PhpSpec\ObjectBehavior;
 use Prophecy\Argument;
 use Prophecy\Doubler\Generator\Node\ArgumentNode;
+use Prophecy\Doubler\Generator\Node\ArgumentTypeNode;
 use Prophecy\Doubler\Generator\Node\ClassNode;
 use Prophecy\Doubler\Generator\Node\MethodNode;
 
@@ -31,6 +32,9 @@ function it_makes_all_constructor_arguments_optional(
         ArgumentNode $arg1,
         ArgumentNode $arg2
     ) {
+        $arg1->getTypeNode()->willReturn(new ArgumentTypeNode('string', 'null'));
+        $arg2->getTypeNode()->willReturn(new ArgumentTypeNode('mixed'));
+
         $class->isExtendable('__construct')->willReturn(true);
         $class->hasMethod('__construct')->willReturn(true);
         $class->getMethod('__construct')->willReturn($method);
@@ -39,6 +43,8 @@ function it_makes_all_constructor_arguments_optional(
         $arg1->setDefault(null)->shouldBeCalled();
         $arg2->setDefault(null)->shouldBeCalled();
 
+        $arg1->setTypeNode(new ArgumentTypeNode('null', 'string'))->shouldBeCalled();
+
         $method->setCode(Argument::type('string'))->shouldBeCalled();
 
         $this->apply($class);
diff --git a/src/Prophecy/Doubler/ClassPatch/DisableConstructorPatch.php b/src/Prophecy/Doubler/ClassPatch/DisableConstructorPatch.php
index 624d94ee..c14b8296 100644
--- a/src/Prophecy/Doubler/ClassPatch/DisableConstructorPatch.php
+++ b/src/Prophecy/Doubler/ClassPatch/DisableConstructorPatch.php
@@ -11,6 +11,7 @@
 
 namespace Prophecy\Doubler\ClassPatch;
 
+use Prophecy\Doubler\Generator\Node\ArgumentTypeNode;
 use Prophecy\Doubler\Generator\Node\ClassNode;
 use Prophecy\Doubler\Generator\Node\MethodNode;
 
@@ -55,6 +56,13 @@ public function apply(ClassNode $node)
         \assert($constructor !== null);
         foreach ($constructor->getArguments() as $argument) {
             $argument->setDefault(null);
+
+            $types = $argument->getTypeNode()->getNonNullTypes();
+            if ([] === $types || ['mixed'] === $types || ['\mixed'] === $types) {
+                continue;
+            }
+
+            $argument->setTypeNode(new ArgumentTypeNode('null', ...$types));
         }
 
         $constructor->setCode(<<<PHP