diff --git a/include/ylt/struct_pack/packer.hpp b/include/ylt/struct_pack/packer.hpp
index 9182fd576..1c7fd34bb 100644
--- a/include/ylt/struct_pack/packer.hpp
+++ b/include/ylt/struct_pack/packer.hpp
@@ -329,7 +329,8 @@ class packer {
                 low_bytes_write_wrapper<size_type>(writer_, size);
               }
               else {
-                static_assert(!sizeof(T), "illegal size_type");
+                std::uint64_t sz = size;
+                low_bytes_write_wrapper<size_type>(writer_, sz);
               }
             }
             else {