diff --git a/generic/pack.cc b/generic/pack.cc
index e92e04c5..f3aa9880 100644
--- a/generic/pack.cc
+++ b/generic/pack.cc
@@ -263,7 +263,8 @@ static void pack_io(Context *ctx)
                 iob = new_cells.back().get();
             }
             packed_cells.insert(ci->name);
-            std::copy(ci->attrs.begin(), ci->attrs.end(), std::inserter(iob->attrs, iob->attrs.begin()));
+            if (iob != nullptr)
+                std::copy(ci->attrs.begin(), ci->attrs.end(), std::inserter(iob->attrs, iob->attrs.begin()));
         }
     }
     for (auto pcell : packed_cells) {