diff --git a/src/graph/graph.hh b/src/graph/graph.hh
index a56d7f48a..8afa570eb 100644
--- a/src/graph/graph.hh
+++ b/src/graph/graph.hh
@@ -653,6 +653,7 @@ struct graph_t
     clone->obj.tail = tail;
     clone->distance = 0;
     clone->space = 0;
+    printf("Creating new node from %p to %p\n", head, tail);
 
     unsigned clone_idx = vertices_.length - 2;
 
diff --git a/src/graph/gsubgpos-graph.hh b/src/graph/gsubgpos-graph.hh
index 984cb2467..5a3321b8e 100644
--- a/src/graph/gsubgpos-graph.hh
+++ b/src/graph/gsubgpos-graph.hh
@@ -104,8 +104,9 @@ struct Lookup : public OT::Lookup
     unsigned type = lookupType;
     unsigned extension_size = OT::ExtensionFormat1<OT::Layout::GSUB_impl::ExtensionSubst>::static_size;
     unsigned start = buffer.length;
-    unsigned end = start + extension_size;
+    unsigned end = start + extension_size - 1;
     if (!buffer.resize (buffer.length + extension_size))
+      // TODO: resizing potentially invalidates existing head/tail pointers.
       return false;
 
     OT::ExtensionFormat1<OT::Layout::GSUB_impl::ExtensionSubst>* extension =
diff --git a/src/graph/serialize.hh b/src/graph/serialize.hh
index ecc6cc5ae..8797ed95a 100644
--- a/src/graph/serialize.hh
+++ b/src/graph/serialize.hh
@@ -205,6 +205,7 @@ inline hb_blob_t* serialize (const graph_t& graph)
 {
   hb_vector_t<char> buffer;
   size_t size = graph.total_size_in_bytes ();
+  printf("Expected graph size: %lu.\n", size);
   if (!buffer.alloc (size)) {
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Unable to allocate output buffer.");
     return nullptr;
diff --git a/src/hb-repacker.hh b/src/hb-repacker.hh
index ea2431a94..041b5260d 100644
--- a/src/hb-repacker.hh
+++ b/src/hb-repacker.hh
@@ -44,7 +44,7 @@ using graph::graph_t;
  */
 
 static inline
-void _make_extensions (hb_tag_t table_tag, graph_t& sorted_graph, hb_vector_t<char>& buffer)
+bool _make_extensions (hb_tag_t table_tag, graph_t& sorted_graph, hb_vector_t<char>& buffer)
 {
   // TODO: Move this into graph or gsubgpos graph?
   hb_hashmap_t<unsigned, graph::Lookup*> lookups;
@@ -52,8 +52,10 @@ void _make_extensions (hb_tag_t table_tag, graph_t& sorted_graph, hb_vector_t<ch
 
   for (auto p : lookups.iter ())
   {
-    p.second->make_extension (table_tag, sorted_graph, p.first, buffer);
+    if (!p.second->make_extension (table_tag, sorted_graph, p.first, buffer))
+      return false;
   }
+  return true;
 }
 
 static inline
@@ -175,6 +177,8 @@ hb_resolve_overflows (const T& packed,
   printf("Resolving overflows!\n");
   graph_t sorted_graph (packed);
   sorted_graph.sort_shortest_distance ();
+  printf("Initial graph size: %lu.\n",
+         sorted_graph.total_size_in_bytes ());
 
   bool will_overflow = graph::will_overflow (sorted_graph);
   if (!will_overflow)
@@ -183,11 +187,17 @@ hb_resolve_overflows (const T& packed,
   }
 
   hb_vector_t<char> extension_buffer;
+  if (!extension_buffer.alloc(100000)) // TODO: cleanup
+    return nullptr;
   if ((table_tag == HB_OT_TAG_GPOS
        ||  table_tag == HB_OT_TAG_GSUB)
       && will_overflow)
   {
-    _make_extensions (table_tag, sorted_graph, extension_buffer);
+    if (!_make_extensions (table_tag, sorted_graph, extension_buffer)) {
+      printf("make extensions failed.\n");
+      return nullptr;
+    }
+
     DEBUG_MSG (SUBSET_REPACK, nullptr, "Assigning spaces to 32 bit subgraphs.");
     if (sorted_graph.assign_spaces ())
       sorted_graph.sort_shortest_distance ();