diff --git a/src/Symfony/Component/VarDumper/Caster/ReflectionCaster.php b/src/Symfony/Component/VarDumper/Caster/ReflectionCaster.php
index e0181ba0ee..6b6d8943b6 100644
--- a/src/Symfony/Component/VarDumper/Caster/ReflectionCaster.php
+++ b/src/Symfony/Component/VarDumper/Caster/ReflectionCaster.php
@@ -183,7 +183,7 @@ class ReflectionCaster
         if (isset($a[$prefix.'returnType'])) {
             $v = $a[$prefix.'returnType'];
             $v = $v instanceof \ReflectionNamedType ? $v->getName() : (string) $v;
-            $a[$prefix.'returnType'] = new ClassStub($a[$prefix.'returnType']->allowsNull() ? '?'.$v : $v, [class_exists($v, false) || interface_exists($v, false) || trait_exists($v, false) ? $v : '', '']);
+            $a[$prefix.'returnType'] = new ClassStub($a[$prefix.'returnType']->allowsNull() && 'mixed' !== $v ? '?'.$v : $v, [class_exists($v, false) || interface_exists($v, false) || trait_exists($v, false) ? $v : '', '']);
         }
         if (isset($a[$prefix.'class'])) {
             $a[$prefix.'class'] = new ClassStub($a[$prefix.'class']);
@@ -323,7 +323,7 @@ class ReflectionCaster
                     if (!$type instanceof \ReflectionNamedType) {
                         $signature .= $type.' ';
                     } else {
-                        if (!$param->isOptional() && $param->allowsNull()) {
+                        if (!$param->isOptional() && $param->allowsNull() && 'mixed' !== $type->getName()) {
                             $signature .= '?';
                         }
                         $signature .= substr(strrchr('\\'.$type->getName(), '\\'), 1).' ';