diff --git a/CI/CI.ml b/CI/CI.ml
index 9c696ce..8661b59 100644
--- a/CI/CI.ml
+++ b/CI/CI.ml
@@ -198,7 +198,8 @@ let make ?(n_states=1) det_space =
     let mo_basis = Ds.mo_basis det_space in
 
     (* While in a sequential region, initiate the parallel
-       4-idx transformation *)
+       4-idx transformation
+       *)
     ignore @@ MOBasis.two_e_ints mo_basis;
 
     let f = 
diff --git a/Parallel_mpi/Farm.ml b/Parallel_mpi/Farm.ml
index cb95481..7da1fbe 100644
--- a/Parallel_mpi/Farm.ml
+++ b/Parallel_mpi/Farm.ml
@@ -210,15 +210,31 @@ let run_parallel_client f =
 
 
 
-let run_parallel ~ordered f stream =
-  match Mpi.comm_rank  Mpi.comm_world with
-  | 0 -> run_parallel_server ~ordered stream
-  | _ -> run_parallel_client f
 
+let run_parallel ~ordered f stream =
+    match Mpi.comm_rank  Mpi.comm_world with
+    | 0 -> run_parallel_server ~ordered stream
+    | _ -> run_parallel_client f
+
+
+let nested = ref false
 
 let run ?(ordered=true) ~f stream =
-  match Mpi.comm_size Mpi.comm_world with
-  | 1 -> run_sequential f stream
-  | _ -> run_parallel ~ordered  f stream
+  if !nested then
+    begin
+      let message =
+        "Nested parallel regions are not supported by Farm.ml"
+      in
+      Printf.eprintf "%s\n%!" message ;
+      exit 1
+    end;
+  nested := true;
+  let result = 
+    match Mpi.comm_size Mpi.comm_world with
+    | 1 -> run_sequential f stream
+    | _ -> run_parallel ~ordered  f stream
+  in
+  nested := false;
+  result