diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 2a68f5467ffd4be27039bfea0bd502746132aa94..0373c39dc6d7e85dd70aabd779006b01fbd6f7e0 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -6,22 +6,11 @@ debian:10 gcc-8-17:
 debian:10 clang-6-libcpp-17:
   image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-clang-6-libcpp-17
   script: duneci-standard-test
-  artifacts:
-    name: "$CI_JOB_STAGE-$CI_COMMIT_REF_NAME"
-    when: on_failure
-    paths:
-    - /tmp/reader_writer_test_ascii32.vtu
-    - /tmp/reader_writer_test_bin32.vtu
-    - /tmp/reader_writer_test_zlib32.vtu
-    - /tmp/reader_writer_test_ascii64.vtu
-    - /tmp/reader_writer_test_bin64.vtu
-    - /tmp/reader_writer_test_zlib64.vtu
-    expire_in: 1 day
 
-#debian:9 gcc-6-14:
-#  image: registry.dune-project.org/docker/ci/dune:2.6-debian-9-gcc-6-14
-#  script: duneci-standard-test
+debian:9 gcc-6-14:
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-9-gcc-6-14
+  script: duneci-standard-test
 
-#ubuntu:18.04 clang-6-17:
-#  image: registry.dune-project.org/docker/ci/dune:2.6-ubuntu-18.04-clang-6-17
-#  script: duneci-standard-test
+ubuntu:18.04 clang-6-17:
+  image: registry.dune-project.org/docker/ci/dune:2.6-ubuntu-18.04-clang-6-17
+  script: duneci-standard-test
diff --git a/dune/vtk/vtkreader.impl.hh b/dune/vtk/vtkreader.impl.hh
index 948c3877211554bd819c567b4abbfb2abee4eba0..c948d846c7dbc654e1c3524d9bb966f789e280dc 100644
--- a/dune/vtk/vtkreader.impl.hh
+++ b/dune/vtk/vtkreader.impl.hh
@@ -22,6 +22,7 @@ void VtkReader<Grid,Creator>::readFromFile (std::string const& filename)
     DUNE_THROW(IOError, "File " << filename << " does not exist!");
 
   std::ifstream input(filename, std::ios_base::in | std::ios_base::binary);
+  assert(input.is_open());
 
   std::string compressor = "";
   std::string data_name = "", data_format = "";
diff --git a/src/test/mixed_element_test.cc b/src/test/mixed_element_test.cc
index 6dc61ef49783918d2c199ceb0ca767e38620e428..589a105eeb02d13ce548038b31f39a293bb1f82f 100644
--- a/src/test/mixed_element_test.cc
+++ b/src/test/mixed_element_test.cc
@@ -71,7 +71,7 @@ void writer_test (GridView const& gridView)
 {
   for (auto const& test_case : test_cases) {
     VtkUnstructuredGridWriter<GridView> vtkWriter(gridView, std::get<1>(test_case), std::get<2>(test_case));
-    vtkWriter.write("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu");
+    vtkWriter.write("mixed_element_test_" + std::get<0>(test_case) + ".vtu");
   }
 }
 
@@ -79,12 +79,12 @@ template <class Grid, class Test>
 void reader_test (Test& test)
 {
   for (auto const& test_case : test_cases) {
-    auto grid = VtkReader<Grid>::read("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu");
+    auto grid = VtkReader<Grid>::read("mixed_element_test_" + std::get<0>(test_case) + ".vtu");
     VtkUnstructuredGridWriter<typename Grid::LeafGridView> vtkWriter(grid->leafGridView(),
       std::get<1>(test_case), std::get<2>(test_case));
-    vtkWriter.write("/tmp/reader_writer_test_" + std::get<0>(test_case) + "_2.vtu");
-    test.check(compare_files("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu",
-                             "/tmp/reader_writer_test_" + std::get<0>(test_case) + "_2.vtu"), std::get<0>(test_case));
+    vtkWriter.write("mixed_element_test_" + std::get<0>(test_case) + "_2.vtu");
+    test.check(compare_files("mixed_element_test_" + std::get<0>(test_case) + ".vtu",
+                             "mixed_element_test_" + std::get<0>(test_case) + "_2.vtu"), std::get<0>(test_case));
   }
 }
 
diff --git a/src/test/reader_writer_test.cc b/src/test/reader_writer_test.cc
index da3e1a1585d31b52176d03f95cf31acda5e8eebb..2421af8caf7106dc3890286c01e1b3ec907d22a8 100644
--- a/src/test/reader_writer_test.cc
+++ b/src/test/reader_writer_test.cc
@@ -71,7 +71,7 @@ void writer_test (GridView const& gridView)
 {
   for (auto const& test_case : test_cases) {
     VtkUnstructuredGridWriter<GridView> vtkWriter(gridView, std::get<1>(test_case), std::get<2>(test_case));
-    vtkWriter.write("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu");
+    vtkWriter.write("reader_writer_test_" + std::get<0>(test_case) + ".vtu");
   }
 }
 
@@ -79,12 +79,12 @@ template <class Grid, class Test>
 void reader_test (Test& test)
 {
   for (auto const& test_case : test_cases) {
-    auto grid = VtkReader<Grid>::read("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu");
+    auto grid = VtkReader<Grid>::read("reader_writer_test_" + std::get<0>(test_case) + ".vtu");
     VtkUnstructuredGridWriter<typename Grid::LeafGridView> vtkWriter(grid->leafGridView(),
       std::get<1>(test_case), std::get<2>(test_case));
-    vtkWriter.write("/tmp/reader_writer_test_" + std::get<0>(test_case) + "_2.vtu");
-    test.check(compare_files("/tmp/reader_writer_test_" + std::get<0>(test_case) + ".vtu",
-                             "/tmp/reader_writer_test_" + std::get<0>(test_case) + "_2.vtu"));
+    vtkWriter.write("reader_writer_test_" + std::get<0>(test_case) + "_2.vtu");
+    test.check(compare_files("reader_writer_test_" + std::get<0>(test_case) + ".vtu",
+                             "reader_writer_test_" + std::get<0>(test_case) + "_2.vtu"));
   }
 }