diff --git a/src/maketools/update-config-txt.sh b/src/maketools/update-config-txt.sh
index 6535f75aea20862a97d26c2d0b682d7c8ec6c493..45998896a2961bd57ce394b58dc934be6676f6d2 100755
--- a/src/maketools/update-config-txt.sh
+++ b/src/maketools/update-config-txt.sh
@@ -103,6 +103,7 @@ echo "$version"
 echo
 echo "# command to lauch mpi processes"
 echo "# syntax: mpiexec command"
+echo "# empty string means that mpiexec was not chosen at configure time"
 echo "mpiexec $mpiexec"
 echo
 echo "# list of 'has' options"