diff --git a/sdcflows/conftest.py b/sdcflows/conftest.py
index e8f2e709be..bfcc77ae86 100644
--- a/sdcflows/conftest.py
+++ b/sdcflows/conftest.py
@@ -72,6 +72,7 @@ def doctest_fixture(doctest_namespace, request):
             dsA_dir=data_dir / "dsA",
             dsB_dir=data_dir / "dsB",
             dsC_dir=data_dir / "dsC",
+            data_dir=data_dir,
         )
         doctest_namespace.update((key, Path(val.root)) for key, val in layouts.items())
 
diff --git a/sdcflows/interfaces/bspline.py b/sdcflows/interfaces/bspline.py
index f2b1dfaa14..4268a18ed5 100644
--- a/sdcflows/interfaces/bspline.py
+++ b/sdcflows/interfaces/bspline.py
@@ -309,12 +309,12 @@ class _ApplyCoeffsFieldInputSpec(BaseInterfaceInputSpec):
     fmap2data_xfm = InputMultiObject(
         File(exists=True),
         desc="the transform by which the target EPI can be resampled on the fieldmap's grid.",
-        xor="data2fmap_xfm",
+        xor=["data2fmap_xfm"],
     )
     data2fmap_xfm = InputMultiObject(
         File(exists=True),
         desc="the transform by which the fieldmap can be resampled on the target EPI's grid.",
-        xor="fmap2data_xfm",
+        xor=["fmap2data_xfm"],
     )
     in_xfms = traits.List(
         traits.List(traits.List(traits.Float)),
@@ -380,6 +380,24 @@ class ApplyCoeffsField(SimpleInterface):
        With those coordinates known, interpolation is trivial.
     4. Generate a spatial image with the new data.
 
+    Example
+    -------
+
+    >>> from sdcflows.interfaces.bspline import ApplyCoeffsField
+    >>> unwarp = ApplyCoeffsField(pe_dir='j', ro_time=0.03125)
+    >>> unwarp.inputs.in_data = str(data_dir / 'epi.nii.gz')
+    >>> unwarp.inputs.in_coeff = str(data_dir / 'topup-coeff.nii.gz')
+    >>> unwarp.inputs.data2fmap_xfm = str(data_dir / 'epi2fmap_xfm.txt')
+    >>> result = unwarp.run()  # doctest: +SKIP
+
+    Inverse transforms may be used instead:
+
+    >>> unwarp = ApplyCoeffsField(pe_dir='j', ro_time=0.03125)
+    >>> unwarp.inputs.in_data = str(data_dir / 'epi.nii.gz')
+    >>> unwarp.inputs.in_coeff = str(data_dir / 'topup-coeff.nii.gz')
+    >>> unwarp.inputs.fmap2data_xfm = str(data_dir / 'fmap2epi_xfm.txt')
+    >>> result = unwarp.run()  # doctest: +SKIP
+
     """
 
     input_spec = _ApplyCoeffsFieldInputSpec
diff --git a/sdcflows/tests/data/epi2fmap_xfm.txt b/sdcflows/tests/data/epi2fmap_xfm.txt
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/sdcflows/tests/data/fmap2epi_xfm.txt b/sdcflows/tests/data/fmap2epi_xfm.txt
new file mode 100644
index 0000000000..e69de29bb2