diff --git a/src/hdmf/backends/hdf5/h5tools.py b/src/hdmf/backends/hdf5/h5tools.py
index 592ffb5fe..5926733d4 100644
--- a/src/hdmf/backends/hdf5/h5tools.py
+++ b/src/hdmf/backends/hdf5/h5tools.py
@@ -1132,6 +1132,7 @@ def write_dataset(self, **kwargs):  # noqa: C901
         data = builder.data
         dataio = None
         options = dict()  # dict with additional
+        # breakpoint()
         if isinstance(data, H5DataIO):
             options['io_settings'] = data.io_settings
             dataio = data
@@ -1488,6 +1489,11 @@ def __list_fill__(cls, parent, name, data, expandable, options=None):
             data_shape = (len(data),)
         else:
             data_shape = get_data_shape(data)
+        # breakpoint()
+        if expandable:
+            # Don't override existing settings
+            if 'maxshape' not in io_settings:
+                io_settings['maxshape'] = tuple([None]*len(data_shape))
 
         # Create the dataset
         try:
diff --git a/src/hdmf/build/objectmapper.py b/src/hdmf/build/objectmapper.py
index b5815ee2c..90cb0654b 100644
--- a/src/hdmf/build/objectmapper.py
+++ b/src/hdmf/build/objectmapper.py
@@ -722,6 +722,7 @@ def build(self, **kwargs):
                 if not isinstance(container, Data):
                     msg = "'container' must be of type Data with DatasetSpec"
                     raise ValueError(msg)
+                breakpoint()
                 spec_dtype, spec_shape, spec_dims, spec = self.__check_dset_spec(self.spec, spec_ext)
                 dimension_labels = self.__get_dimension_labels_from_spec(container.data, spec_shape, spec_dims)
                 if isinstance(spec_dtype, RefSpec):