diff --git a/src/davidson_keywords/usef.irp.f b/src/davidson_keywords/usef.irp.f
index fed2ba9b..7ca2d203 100644
--- a/src/davidson_keywords/usef.irp.f
+++ b/src/davidson_keywords/usef.irp.f
@@ -13,7 +13,9 @@ BEGIN_PROVIDER [ integer, nthreads_davidson ]
  character*(32) :: env
  call getenv('QP_NTHREADS_DAVIDSON',env)
  if (trim(env) /= '') then
+   call lock_io
    read(env,*) nthreads_davidson
+   call unlock_io
    call write_int(6,nthreads_davidson,'Target number of threads for <Psi|H|Psi>')
  endif
 END_PROVIDER