diff --git a/src/Php/AstNodeType.php b/src/Php/AstNodeType.php
index 69020ae9..e27b5140 100644
--- a/src/Php/AstNodeType.php
+++ b/src/Php/AstNodeType.php
@@ -142,4 +142,9 @@ trait AstNodeType
     {
         return $expr instanceof Expr\Array_ && count($expr->items) === 0;
     }
+
+    protected function isNull(NodeAbstract $expr): bool
+    {
+        return $expr instanceof Node\Expr\ConstFetch && $expr->name->toString() === 'null';
+    }
 }
diff --git a/src/Php/CompilerBase.php b/src/Php/CompilerBase.php
index f7461930..3bd7252d 100644
--- a/src/Php/CompilerBase.php
+++ b/src/Php/CompilerBase.php
@@ -1055,11 +1055,14 @@ class CompilerBase extends \PhpAot\Core\Translator
             }
             if ($param->default) {
                 if ($param->byRef) {
-                    if (!$this->isEmptyArray($param->default)) {
-                        $this->fatalError($param, 'Default value for parameters passed by reference must be an empty array');
-                    } else {
+                    if ($this->isEmptyArray($param->default)) {
                         $argInfo->default = 'php::getEmptyArrayRef()';
                         $argInfo->defaultValue = null;
+                    } elseif ($this->isNull($param->default)) {
+                        $argInfo->default = 'nullptr';
+                        $argInfo->defaultValue = null;
+                    } else {
+                        $this->fatalError($param, 'Only null and empty array can be used as default value for reference parameter');
                     }
                 } else {
                     $argInfo->default = $this->parseParamDefaultValue($param->default);
diff --git a/src/Php/FuncCallOptimizer.php b/src/Php/FuncCallOptimizer.php
index 00f74a06..216069b5 100644
--- a/src/Php/FuncCallOptimizer.php
+++ b/src/Php/FuncCallOptimizer.php
@@ -12,6 +12,42 @@ use PhpParser\Node;
 
 trait FuncCallOptimizer
 {
+    public function genFuncGetArgs(string $name, Node\Expr\FuncCall $expr): string
+    {
+        $funcDef = $this->functionDef;
+        $list = [];
+        foreach ($funcDef->argInfoList as $i => $argInfo) {
+            if ($argInfo->variadic) {
+                $tmpVar = $this->addTmpVar(self::TYPE_ARRAY);
+                $this->context->beforeStmtLines[] = $this->genArray($list) . ';';
+                $this->context->beforeStmtLines[] = $tmpVar . '.merge(' . $argInfo->name . ');';
+                return $tmpVar;
+            }
+            $list[] = $argInfo->name;
+        }
+        return $this->genArray($list);
+    }
+
+    public function genFuncGetArg(string $name, Node\Expr\FuncCall $expr)
+    {
+        $position = $expr->args[0]->value;
+        if ($this->isScalarInt($position)) {
+            $funcDef = $this->functionDef;
+            $posInt = intval($position->value);
+            foreach ($funcDef->argInfoList as $i => $argInfo) {
+                if ($argInfo->variadic) {
+                    return $argInfo->name . '.offsetGet(' . ($posInt - $i) . ')';
+                }
+                if ($i == $posInt) {
+                    return $argInfo->name;
+                }
+            }
+            $this->fatalError($expr, 'wrong parameter position `' . $posInt . '`');
+        } else {
+            $this->fatalError($expr, 'func_get_arg() only support scalar int');
+        }
+    }
+
     protected function parseFuncCallWithOptimizer(string $name, Node\Expr\FuncCall $expr): string|false
     {
         $getArg = function ($i) use ($expr) {
@@ -71,41 +107,6 @@ trait FuncCallOptimizer
         return false;
     }
 
-    public function genFuncGetArgs(string $name, Node\Expr\FuncCall $expr): string
-    {
-        $funcDef = $this->functionDef;
-        $list = [];
-        foreach ($funcDef->argInfoList as $i => $argInfo) {
-            if ($argInfo->variadic) {
-                $tmpVar = $this->addTmpVar(self::TYPE_ARRAY);
-                $this->context->beforeStmtLines[] = $this->genArray($list) . ';';
-                $this->context->beforeStmtLines[] = $tmpVar . '.merge(' . $argInfo->name . ');';
-                return $tmpVar;
-            }
-            $list[] = $argInfo->name;
-        }
-        return $this->genArray($list);
-    }
-
-    public function genFuncGetArg(string $name, Node\Expr\FuncCall $expr)
-    {
-        $position = $expr->args[0]->value;
-        if ($this->isScalarInt($position)) {
-            $funcDef = $this->functionDef;
-            $posInt = intval($position->value);
-            foreach ($funcDef->argInfoList as $i => $argInfo) {
-                if ($argInfo->variadic) {
-                    return $argInfo->name . '.offsetGet(' . ($posInt - $i) . ')';
-                } elseif ($i == $posInt) {
-                    return $argInfo->name;
-                }
-            }
-            $this->fatalError($expr, 'wrong parameter position `' . $posInt . '`');
-        } else {
-            $this->fatalError($expr, 'func_get_arg() only support scalar int');
-        }
-    }
-
     protected function genFuncNumArgs(string $name, Node\Expr\FuncCall $expr): string
     {
         $funcDef = $this->functionDef;
diff --git a/src/Php/Preprocessor.php b/src/Php/Preprocessor.php
index 4b95e3bf..236588bf 100644
--- a/src/Php/Preprocessor.php
+++ b/src/Php/Preprocessor.php
@@ -106,7 +106,7 @@ class Preprocessor extends CompilerBase
             $type = $v->getType();
             switch ($type) {
                 case 'Stmt_Namespace':
-                    $this->prepareNamespaceDef($v);
+                    $this->prepareNamespace($v);
                     break;
                 case 'Stmt_Enum':
                 case 'Stmt_Class':
@@ -161,7 +161,7 @@ class Preprocessor extends CompilerBase
         $this->symbolCallInFile[$this->file] = array_unique($this->symbolCallInFile[$this->file]);
     }
 
-    protected function prepareNamespaceDef(Node\Stmt\Namespace_ $node): void
+    protected function prepareNamespace(Node\Stmt\Namespace_ $node): void
     {
         $this->resetNamespace();
         $this->namespace = $node->name ? $this->parseIdentifier($node->name) : '';
diff --git a/src/Php/Translator.php b/src/Php/Translator.php
index ac41a099..9a37973b 100644
--- a/src/Php/Translator.php
+++ b/src/Php/Translator.php
@@ -17,6 +17,7 @@ use PhpAot\Php\Entity\InterfaceDef;
 use PhpAot\Php\Entity\MethodDef;
 use PhpAot\Php\Entity\PropertyDef;
 use PhpAot\Php\Exception\Redo;
+use PhpAot\Php\Exception\Unsupported;
 use PhpParser\Modifiers;
 use PhpParser\Node;
 use PhpParser\Node\Stmt\Foreach_;
@@ -825,6 +826,9 @@ class Translator extends Preprocessor
                 case 'Stmt_Interface':
                     $code .= $this->parseInterface($v2) . PHP_EOL;
                     break;
+                case 'Stmt_Trait':
+                    $code .= $this->parseTrait($v2) . PHP_EOL;
+                    break;
                 default:
                     abort($v2);
             }
@@ -1363,4 +1367,9 @@ class Translator extends Preprocessor
 
         return $code;
     }
+
+    protected function parseTrait(Node\Stmt\Trait_ $trait)
+    {
+        throw new Unsupported('Unsupported Trait ');
+    }
 }