diff --git a/App/lib/core/Gen/GenApi.vp b/App/lib/core/Gen/GenApi.vp
index b645464..2f124ba 100644
--- a/App/lib/core/Gen/GenApi.vp
+++ b/App/lib/core/Gen/GenApi.vp
@@ -377,10 +377,16 @@ def _WillFreeArgIndex(fn: ast.FunctionDef*, arg: ast.AST*) -> int:
     if arg is None:
         fatal_error(arg, "@willFree(...) requires a parameter name or its 0-based position", "LifetimeError")
         return -1
-    if fn.args is None or fn.args.args is None:
+    # FunctionDef.args 的字段静态类型是 AST*（见 includes/ast/stmts.vp），必须先转成 Arguments*
+    # 才能再取 .args 列表；直接写 fn.args.args 会去 AST 类查 'args' 触发 MemberLookupFailFast。
+    if fn.args is None:
         fatal_error(arg, "@willFree(...) cannot be used here: the definition has no parameter list", "LifetimeError")
         return -1
-    pl: list[ast.AST*]* = fn.args.args
+    fargs: ast.Arguments* = ast.Arguments*(fn.args)
+    if fargs is None or fargs.args is None:
+        fatal_error(arg, "@willFree(...) cannot be used here: the definition has no parameter list", "LifetimeError")
+        return -1
+    pl: list[ast.AST*]* = fargs.args
     pn: sizet = len(pl)
     if arg.kind() == ast.ASTKind.Name:
         nma: ast.Name* = ast.Name*(arg)