Fix constness.

diff --git a/source/lac/petsc_parallel_block_sparse_matrix.cc b/source/lac/petsc_parallel_block_sparse_matrix.cc
index 49911c8dee8bca8cd20575e7e6c92b172c478935..4d0accae14f251e329e3a960b02eeca8d784cb7d 100644 (file)
--- a/source/lac/petsc_parallel_block_sparse_matrix.cc
+++ b/source/lac/petsc_parallel_block_sparse_matrix.cc
@@ -176,7 +176,7 @@ namespace PETScWrappers
       return block(0, 0).get_mpi_communicator();
     }
 
-    BlockSparseMatrix::operator Mat() const
+    BlockSparseMatrix::operator const Mat &() const
     {
       return petsc_nest_matrix;
     }