diff --git a/graph/Graph.cc b/graph/Graph.cc
index 46833f82..512750f4 100644
--- a/graph/Graph.cc
+++ b/graph/Graph.cc
@@ -515,9 +515,14 @@ Graph::makeArrivals(Vertex *vertex,
 }
 
 Arrival *
-Graph::arrivals(Vertex *vertex) const
+Graph::arrivals(Vertex *vertex)
 {
-  return arrivals_.pointer(vertex->arrivals());
+  Arrival *arrivals;
+  {
+    UniqueLock lock(arrivals_lock_);
+    arrivals = arrivals_.pointer(vertex->arrivals());
+  }
+  return arrivals;
 }
 
 void
diff --git a/include/sta/Graph.hh b/include/sta/Graph.hh
index 32141e7b..3d989bdb 100644
--- a/include/sta/Graph.hh
+++ b/include/sta/Graph.hh
@@ -100,7 +100,7 @@ public:
   VertexId vertexCount() { return vertices_->size(); }
   Arrival *makeArrivals(Vertex *vertex,
 			uint32_t count);
-  Arrival *arrivals(Vertex *vertex) const;
+  Arrival *arrivals(Vertex *vertex);
   void clearArrivals();
   PathVertexRep *makePrevPaths(Vertex *vertex,
 			       uint32_t count);
@@ -274,7 +274,7 @@ public:
   bool isRoot() const{ return level_ == 0; }
   LevelColor color() const { return static_cast<LevelColor>(color_); }
   void setColor(LevelColor color);
-  ArrivalId arrivals() const { return arrivals_; }
+  ArrivalId arrivals() { return arrivals_; }
   void setArrivals(ArrivalId id);
   PrevPathId prevPaths() const { return prev_paths_; }
   void setPrevPaths(PrevPathId id);
diff --git a/search/PathVertexRep.cc b/search/PathVertexRep.cc
index 1411bc01..8b338a70 100644
--- a/search/PathVertexRep.cc
+++ b/search/PathVertexRep.cc
@@ -132,7 +132,7 @@ PathVertexRep::tag(const StaState *sta) const
 Arrival
 PathVertexRep::arrival(const StaState *sta) const
 { 
-  const Graph *graph = sta->graph();
+  Graph *graph = sta->graph();
   const Search *search = sta->search();
   Tag *tag = search->tag(tag_index_);
   Vertex *vertex = graph->vertex(vertex_id_);