diff --git a/include/sta/Search.hh b/include/sta/Search.hh
index dc73d6ed..a2430809 100644
--- a/include/sta/Search.hh
+++ b/include/sta/Search.hh
@@ -606,6 +606,7 @@ protected:
   ExceptionFrom *filter_from_;
   ExceptionTo *filter_to_;
   VertexSet *filtered_arrivals_;
+  std::mutex filtered_arrivals_lock_;
   bool found_downstream_clk_pins_;
   PathGroups *path_groups_;
   VisitPathEnds *visit_path_ends_;
diff --git a/search/Search.cc b/search/Search.cc
index 2eefcd4a..242e05b9 100644
--- a/search/Search.cc
+++ b/search/Search.cc
@@ -2686,8 +2686,10 @@ Search::setVertexArrivals(Vertex *vertex,
       }
       tag_bldr->copyArrivals(tag_group, prev_arrivals, prev_paths);
       vertex->setTagGroupIndex(tag_group->index());
-      if (tag_group->hasFilterTag())
+      if (tag_group->hasFilterTag()) {
+        LockGuard lock(this->filtered_arrivals_lock_);
         filtered_arrivals_->insert(vertex);
+      }
 
       if (has_requireds) {
 	requiredInvalid(vertex);
@@ -2712,8 +2714,10 @@ Search::setVertexArrivals(Vertex *vertex,
       tag_bldr->copyArrivals(tag_group, arrivals, prev_paths);
 
       vertex->setTagGroupIndex(tag_group->index());
-      if (tag_group->hasFilterTag())
+      if (tag_group->hasFilterTag()) {
+        LockGuard lock(this->filtered_arrivals_lock_);
         filtered_arrivals_->insert(vertex);
+      }
     }
   }
 }