diff --git a/l4/pkg/libkproxy/lib/src/factory_svr.cc b/l4/pkg/libkproxy/lib/src/factory_svr.cc
index 8faf423..126da8d 100644
--- a/l4/pkg/libkproxy/lib/src/factory_svr.cc
+++ b/l4/pkg/libkproxy/lib/src/factory_svr.cc
@@ -46,12 +46,14 @@ public:
   static int handle_task(Factory_svr *svr, Factory_interface *fi,
                          L4::Ipc::Iostream &ios)
     {
-      l4_fpage_t utcb_area;
+      L4::Ipc::Varg utcb_area;
+      ios.get(&utcb_area);
+      if (!utcb_area.is_of<l4_fpage_t>())
+        return -L4_EINVAL;
       L4::Cap<L4::Task> t = svr->cap_alloc<L4::Task>();
       if (!t.is_valid())
         return -L4_ENOMEM;
-      ios >> utcb_area.raw;
-      int r = fi->create_task(t, utcb_area);
+      int r = fi->create_task(t, utcb_area.value<l4_fpage_t>());
       if (r == 0)
         ios << t;
       return r;