diff --git a/src/dag_s.f90 b/src/dag_s.f90
index f7613a1..5e9bb53 100644
--- a/src/dag_s.f90
+++ b/src/dag_s.f90
@@ -39,7 +39,7 @@ pure module function topological_sort(dag) result(order)
 
       searched_and_ordered = searched_and_ordered_t(s = [integer::], o = [integer::])
 
-      do concurrent(v = 1:size(dag%vertices))
+      do v = 1, size(dag%vertices)
         if (.not. any(v == searched_and_ordered%s)) &
           searched_and_ordered = depth_first_search(v, [integer::], searched_and_ordered%o)
       end do
@@ -60,7 +60,7 @@ pure recursive function depth_first_search(v, d, o) result(hybrid)
       associate(dependencies => dag%depends_on(v))
         block
           integer w
-          do concurrent(w = 1:size(dependencies))
+          do w = 1, size(dependencies)
             associate(w_dependencies => dependencies(w))
               if (.not. any(w_dependencies == hybrid%s)) hybrid = depth_first_search(w_dependencies, [d, v], hybrid%o)
             end associate