diff --git a/xcp_d/tests/test_interfaces_bids.py b/xcp_d/tests/test_interfaces_bids.py
index 36c760038..7d9e35ab4 100644
--- a/xcp_d/tests/test_interfaces_bids.py
+++ b/xcp_d/tests/test_interfaces_bids.py
@@ -1,4 +1,6 @@
 """Tests for framewise displacement calculation."""
+from nipype.interfaces.base import isdefined
+
 from xcp_d.interfaces import bids
 
 
@@ -34,4 +36,4 @@ def test_infer_bids_uris():
         dataset_path=dataset_path,
     )
     out = infer_bids_uris.run()
-    assert out.outputs.bids_uris == []
+    assert not isdefined(out.outputs.bids_uris)
diff --git a/xcp_d/tests/test_utils_utils.py b/xcp_d/tests/test_utils_utils.py
index f9002ad5f..5b649d603 100644
--- a/xcp_d/tests/test_utils_utils.py
+++ b/xcp_d/tests/test_utils_utils.py
@@ -416,11 +416,11 @@ def test_listify():
     ]
     outputs = [
         [1],
-        [1],
+        (1,),
         ["a"],
         ["a"],
         ["a", ["b", "c"]],
-        ["a", "b"],
+        ("a", "b"),
     ]
     for i, input_ in enumerate(inputs):
         expected_output = outputs[i]
diff --git a/xcp_d/utils/bids.py b/xcp_d/utils/bids.py
index 646161a2c..5eb0e5f6c 100644
--- a/xcp_d/utils/bids.py
+++ b/xcp_d/utils/bids.py
@@ -986,7 +986,7 @@ def _make_uri(in_file, dataset_name, dataset_path):
     ValueError
         If ``in_file`` is not relative to ``dataset_path``.
     """
-    bids_uri = [f"bids:{dataset_name}:{str(Path(in_file).relative_to(dataset_path))}"]
+    bids_uri = f"bids:{dataset_name}:{str(Path(in_file).relative_to(dataset_path))}"
     return bids_uri
 
 
@@ -1001,7 +1001,7 @@ def _make_xcpd_uri(out_file, output_dir):
     if isinstance(out_file, list):
         return [_make_uri(of, "xcp_d", dataset_path) for of in out_file]
     else:
-        return _make_uri(out_file, "xcp_d", dataset_path)
+        return [_make_uri(out_file, "xcp_d", dataset_path)]
 
 
 def _make_preproc_uri(out_file, fmri_dir):
@@ -1011,7 +1011,7 @@ def _make_preproc_uri(out_file, fmri_dir):
     if isinstance(out_file, list):
         return [_make_uri(of, "preprocessed", fmri_dir) for of in out_file]
     else:
-        return _make_uri(out_file, "preprocessed", fmri_dir)
+        return [_make_uri(out_file, "preprocessed", fmri_dir)]
 
 
 def _make_custom_uri(out_file):
@@ -1023,4 +1023,4 @@ def _make_custom_uri(out_file):
     if isinstance(out_file, list):
         return [_make_uri(of, "custom_confounds", os.path.dirname(of)) for of in out_file]
     else:
-        return _make_uri(out_file, "custom_confounds", os.path.dirname(out_file))
+        return [_make_uri(out_file, "custom_confounds", os.path.dirname(out_file))]
diff --git a/xcp_d/utils/utils.py b/xcp_d/utils/utils.py
index a2c1a7777..f40ffdaf5 100644
--- a/xcp_d/utils/utils.py
+++ b/xcp_d/utils/utils.py
@@ -547,6 +547,8 @@ def _make_dictionary(metadata=None, **kwargs):
     """
     from copy import deepcopy
 
+    from xcp_d.utils.utils import _listify
+
     if metadata:
         out_metadata = deepcopy(metadata)
         for key, value in kwargs.items():
@@ -554,7 +556,7 @@ def _make_dictionary(metadata=None, **kwargs):
                 out_metadata[key] = value
             elif isinstance(value, list):
                 # Append the values if they're a list
-                out_metadata[key] += value
+                out_metadata[key] = _listify(out_metadata[key]) + value
             else:
                 # Overwrite the old value
                 out_metadata[key] = value