MF::check_vector_compatibility(): improve asser message

diff --git a/include/deal.II/matrix_free/vector_access_internal.h b/include/deal.II/matrix_free/vector_access_internal.h
index 1874593d6122188f6252245630cd99d9957ea105..ce6c281e2249d8b9869db89cc7b07954ab891f60 100644 (file)
--- a/include/deal.II/matrix_free/vector_access_internal.h
+++ b/include/deal.II/matrix_free/vector_access_internal.h
@@ -201,8 +201,11 @@ namespace internal
            ExcMessage(
              "The parallel layout of the given vector is not "
              "compatible with the parallel partitioning in MatrixFree. "
-             "Use MatrixFree::initialize_dof_vector to get a "
-             "compatible vector."));
+             "A potential reason is that you did not use "
+             "MatrixFree::initialize_dof_vector() to get a "
+             "compatible vector. If you did, the dof_handler_index "
+             "used there and the one passed to the constructor of "
+             "FEEvaluation do not match."));
   }