diff --git a/nitransforms/base.py b/nitransforms/base.py
index fa05f1f6..6e1634c6 100644
--- a/nitransforms/base.py
+++ b/nitransforms/base.py
@@ -10,7 +10,6 @@
 
 from pathlib import Path
 import numpy as np
-import h5py
 import warnings
 from nibabel.loadsave import load as _nbload
 from nibabel import funcs as _nbfuncs
@@ -333,15 +332,9 @@ def map(self, x, inverse=False):
         """
         return x
 
-    def to_filename(self, filename, fmt="X5"):
+    def to_filename(self, filename, fmt="X5", moving=None, x5_inverse=False):
         """Store the transform in BIDS-Transforms HDF5 file format (.x5)."""
-        with h5py.File(filename, "w") as out_file:
-            out_file.attrs["Format"] = "X5"
-            out_file.attrs["Version"] = np.uint16(1)
-            root = out_file.create_group("/0")
-            self._to_hdf5(root)
-
-        return filename
+        raise NotImplementedError
 
     def _to_hdf5(self, x5_root):
         """Serialize this object into the x5 file format."""
diff --git a/nitransforms/tests/test_base.py b/nitransforms/tests/test_base.py
index 49d7f7af..45611745 100644
--- a/nitransforms/tests/test_base.py
+++ b/nitransforms/tests/test_base.py
@@ -108,7 +108,8 @@ def _to_hdf5(klass, x5_root):
         _ = xfm.ndim
 
     # Test to_filename
-    xfm.to_filename("data.x5")
+    with pytest.raises(NotImplementedError):
+        xfm.to_filename("data.x5")
 
     # Test identity transform
     xfm = nitl.Affine()