diff --git a/cobaya/output.py b/cobaya/output.py
index 879539085..37c582ffc 100644
--- a/cobaya/output.py
+++ b/cobaya/output.py
@@ -89,6 +89,11 @@ def lock_error(self):
                     pass
             except OSError:
                 pass
+        if mpi.is_disabled():
+            raise LoggedError(self.log,
+                              "File %s is locked by another process, you are running "
+                              "with MPI disabled but may have more than one process. "
+                              "Note that --test should not be used with MPI.")
         if mpi.get_mpi():
             import mpi4py
         else: