diff --git a/.ci/ubuntu25.04_py3/Dockerfile b/.ci/ubuntu25.04_py3/Dockerfile
new file mode 100644
index 00000000..eb0bb8c2
--- /dev/null
+++ b/.ci/ubuntu25.04_py3/Dockerfile
@@ -0,0 +1,45 @@
+FROM ubuntu:25.04
+
+MAINTAINER DESY, Jan Kotanski <jankotan@gmail.com>
+
+
+RUN apt-get update
+RUN apt-get install -y base-passwd apt-utils dialog -y
+RUN echo 'debconf debconf/frontend select Noninteractive' | debconf-set-selections
+RUN apt-get -qq update && apt-get -qq install -y libterm-readline-gnu-perl software-properties-common coreutils gnupg2 procps curl apt-transport-https gnupg2 ca-certificates wget gpg-agent
+
+RUN curl -s http://repos.pni-hdri.de/debian_repo.pub.gpg  | gpg --no-default-keyring --keyring gnupg-ring:/etc/apt/trusted.gpg.d/debian-hdri-repo.gpg --import
+RUN chmod 644 /etc/apt/trusted.gpg.d/debian-hdri-repo.gpg
+RUN cd /etc/apt/sources.list.d && wget http://repos.pni-hdri.de/plucky-pni-hdri.list
+
+RUN apt-get -qq update && export DEBIAN_FRONTEND=noninteractive && apt-get -qq dist-upgrade
+RUN apt-get -qq update && export DEBIAN_FRONTEND=noninteractive && apt-get -qq install -y python3-sphinx apt-utils net-tools
+RUN apt-get -qq install -y python3-numpy libhdf5-dev libboost-regex-dev libboost-program-options-dev libboost-system-dev libboost-date-time-dev libboost-filesystem-dev libblas-dev libatlas-base-dev cython3 python3-setuptools libboost-python-dev python3-numpy-dev g++ python3-h5py
+RUN apt-get -qq install -y hdf5-filter-plugin bitshuffle-serial-links python3-pytest git cmake doxygen python3-sphinx libboost-all-dev
+RUN apt-get -qq install -y libpninexus3.2.0-dev libpninexus3.2.0 libh5cpp0.6.1-dev libh5cpp0.6.1
+RUN useradd -ms /bin/bash tango
+
+# # install latest h5cpp
+# RUN mkdir -p /opt/h5cpp
+# RUN mkdir -p /h5cpp-src
+# RUN cd /h5cpp-src && git clone https://github.com/ess-dmsc/h5cpp && mkdir build
+# RUN cd /h5cpp-src/build && cmake -DCMAKE_INSTALL_PREFIX=/opt/h5cpp -DCMAKE_BUILD_TYPE=Release -DH5CPP_CONAN=DISABLE -DH5CPP_DISABLE_TESTS=True  -DH5CPP_WITH_BOOST=OFF ../h5cpp && make install -j4
+# ENV CPLUS_INCLUDE_PATH=/opt/h5cpp/include
+# ENV LD_LIBRARY_PATH=/opt/h5cpp/lib
+# ENV H5CPP=/opt/h5cpp/lib/cmake/h5cpp-0.6
+# ENV H5CPP_LOCAL_PATH=/opt/h5cpp
+# ENV HDF5_HL_LOCAL_PATH=/usr/lib/x86_64-linux-gnu/hdf5/serial/
+
+# # install latest linpninexus
+# RUN mkdir -p /opt/libpninexus
+# RUN mkdir -p /libpninexus-src
+# RUN cd /libpninexus-src && git clone https://github.com/pni-libraries/libpninexus && mkdir build
+# # RUN cd /libpninexus-src/build && cmake -Dh5cpp_DIR=$H5CPP  -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/opt/h5cpp/   -DPNINEXUS_CONAN=DISABLE ../libpninexus  && make -j4 && make install -j4
+# RUN cd /libpninexus-src/build && cmake  -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/opt/libpninexus/   -DPNINEXUS_CONAN=DISABLE ../libpninexus  && make -j4 && make install -j4
+# ENV CPLUS_INCLUDE_PATH=$CPLUS_INCLUDE_PATH:/opt/libpninexus/include
+# ENV LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/opt/libpninexus/lib
+# ENV PNINEXUS_LOCAL_PATH=/opt/libpninexus
+
+ENV PKG_CONFIG_PATH=/home/tango/lib/pkgconfig
+USER tango
+WORKDIR /home/tango
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 5c67e983..416b98fb 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -24,7 +24,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        os: [debian12, debian11, debian10, ubuntu24.10, ubuntu24.04, ubuntu22.04]
+        os: [debian12, debian11, debian10, ubuntu25.04, ubuntu24.04, ubuntu22.04]
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
diff --git a/doc/sphinx/h5cpp/datatype.rst b/doc/sphinx/h5cpp/datatype.rst
index 292b7744..142ce710 100644
--- a/doc/sphinx/h5cpp/datatype.rst
+++ b/doc/sphinx/h5cpp/datatype.rst
@@ -10,7 +10,7 @@ Enumerations
 
 .. autosummary::
 
-   pninexus.h5cpp.datatype.Class
+   Class
    Order
    Sign
    Norm
diff --git a/src/cpp/h5cpp/attribute/attribute.cpp b/src/cpp/h5cpp/attribute/attribute.cpp
index 71846b9e..5e766a15 100644
--- a/src/cpp/h5cpp/attribute/attribute.cpp
+++ b/src/cpp/h5cpp/attribute/attribute.cpp
@@ -115,7 +115,7 @@ void attribute_write(const hdf5::attribute::Attribute &self,
   Datatype mem_type = hdf5::datatype::create<numpy::ArrayAdapter>(array_adapter);
   if(has_variable_length_string_type(self) &&
       (mem_type.get_class() == hdf5::datatype::Class::String))
-    mem_type = String::variable();
+    mem_type = self.datatype();
   if((self.datatype().get_class() == hdf5::datatype::Class::Enum) &&
      hdf5::datatype::is_bool(hdf5::datatype::Enum(self.datatype())))
     mem_type = hdf5::datatype::create<hdf5::datatype::EBool>();
diff --git a/src/cpp/h5cpp/node/dataset.cpp b/src/cpp/h5cpp/node/dataset.cpp
index b089c561..6e2ddf0f 100644
--- a/src/cpp/h5cpp/node/dataset.cpp
+++ b/src/cpp/h5cpp/node/dataset.cpp
@@ -51,11 +51,11 @@ void dataset_write_chunk(const hdf5::node::Dataset &self,
 			 hdf5::property::DatasetTransferList())
 {
   numpy::ArrayAdapter array_adapter(data);
-  std::vector<long long unsigned int> voffset;
+  std::vector<hsize_t> voffset;
 
   for (boost::python::ssize_t i = 0, end = len(offset); i < end; ++i){
     boost::python::object o = offset[i];
-    boost::python::extract<long long unsigned int> s(o);
+    boost::python::extract<hsize_t> s(o);
     if (s.check()){
       voffset.push_back(s());
     }
@@ -73,12 +73,12 @@ std::uint32_t dataset_read_chunk(const hdf5::node::Dataset &self,
 				 hdf5::property::DatasetTransferList())
 {
   numpy::ArrayAdapter array_adapter(data);
-  std::vector<long long unsigned int> voffset;
+  std::vector<hsize_t> voffset;
 
 
   for (boost::python::ssize_t i = 0, end = len(offset); i < end; ++i){
     boost::python::object o = offset[i];
-    boost::python::extract<long long unsigned int> s(o);
+    boost::python::extract<hsize_t> s(o);
     if (s.check()){
       voffset.push_back(s());
     }
@@ -87,14 +87,14 @@ std::uint32_t dataset_read_chunk(const hdf5::node::Dataset &self,
   return self.read_chunk(array_adapter,voffset,dtpl);
 }
 
-long long unsigned int dataset_chunk_storage_size(const hdf5::node::Dataset &self,
+hsize_t dataset_chunk_storage_size(const hdf5::node::Dataset &self,
 						  boost::python::list offset)
 {
-  std::vector<long long unsigned int> voffset;
+  std::vector<hsize_t> voffset;
 
   for (boost::python::ssize_t i = 0, end = len(offset); i < end; ++i){
     boost::python::object o = offset[i];
-    boost::python::extract<long long unsigned int> s(o);
+    boost::python::extract<hsize_t> s(o);
     if (s.check()){
       voffset.push_back(s());
     }
diff --git a/src/pninexus/h5cpp/node/__init__.py b/src/pninexus/h5cpp/node/__init__.py
index ba44f2dc..4a743a1f 100644
--- a/src/pninexus/h5cpp/node/__init__.py
+++ b/src/pninexus/h5cpp/node/__init__.py
@@ -302,7 +302,7 @@ def dataset_write(self, data, selection=None):
 
     if isinstance(self.datatype, datatype.String):
         if self.datatype.is_variable_length:
-            memory_type = datatype.String.variable()
+            memory_type = self.datatype
 
     # if the data is bool numpy array we have to convert it to a
     # int array