diff --git a/src/problem/Formulation.cpp b/src/problem/Formulation.cpp
index c8576b20ee5ca88a508a186f027ffa1c3578653c..2279bab5542b6c873147c63f1171deeb88bbc153 100644
--- a/src/problem/Formulation.cpp
+++ b/src/problem/Formulation.cpp
@@ -897,6 +897,7 @@ namespace gmshddm
       MPI_Barrier(PETSC_COMM_WORLD);
 
       // get the local interface unknowns for each processor
+      _rhs.clear();
       for(auto idom = 0ULL; idom < _surface.size(); ++idom) {
         if(mpi::isItMySubdomain(idom)) {
           for(auto it = _surface[idom].begin(); it != _surface[idom].end(); ++it) {