diff --git a/dump2sq.c b/dump2sq.c
index a27c30b..f9de1f4 100644
--- a/dump2sq.c
+++ b/dump2sq.c
@@ -248,8 +248,11 @@ void CalcCn_pp(DATA *data, HEAD *head, TYPE *type, DATASET *cn,
       if (cn->xmin < d && d < cn->xmax){
 	p = (int)floor((d - cn->xmin)/cn->dx);
 	k = type->pairid[data[i].type_new][data[j].type_new];
+	#pragma omp critical
+	{
 	cn_[p][k] = cn_[p][k] + 1.0;
 	cn_[p][cn->m-1] = cn_[p][cn->m-1] + 1.0;
+	}
       }
     }
   }