diff --git a/RecoVertex/KalmanVertexFit/interface/SimpleVertexTree.h b/RecoVertex/KalmanVertexFit/interface/SimpleVertexTree.h
index 792a68423c350..7947ed202d12c 100644
--- a/RecoVertex/KalmanVertexFit/interface/SimpleVertexTree.h
+++ b/RecoVertex/KalmanVertexFit/interface/SimpleVertexTree.h
@@ -106,6 +106,7 @@ class SimpleVertexTree {
   int maxTrack;
   TString* parameterNames[5];
   TrackAssociatorByChi2 * associatorForParamAtPca;
+  std::atomic<int> nFill{0};
 };
 #endif
 
diff --git a/RecoVertex/KalmanVertexFit/src/SimpleVertexTree.cc b/RecoVertex/KalmanVertexFit/src/SimpleVertexTree.cc
index 31089fbc9a91b..0ec3891bf5dbe 100644
--- a/RecoVertex/KalmanVertexFit/src/SimpleVertexTree.cc
+++ b/RecoVertex/KalmanVertexFit/src/SimpleVertexTree.cc
@@ -172,7 +172,6 @@ void SimpleVertexTree::fill()
   //
   // fill entry
   //
-  static int nFill(0);
   vertexTree->GetDirectory()->cd();
   vertexTree->Fill();
   if ( (++nFill)%1000==0 )  vertexTree->AutoSave();