diff --git a/xcp_d/utils/utils.py b/xcp_d/utils/utils.py
index 5f7640f03..42de61d43 100644
--- a/xcp_d/utils/utils.py
+++ b/xcp_d/utils/utils.py
@@ -563,3 +563,8 @@ def _make_dictionary(metadata=None, **kwargs):
 def _out_file_to_source(in_file, dataset_name, dataset_path):
     bids_uri = [f"bids:{dataset_name}:{str(Path(in_file).relative_to(dataset_path))}"]
     return bids_uri
+
+
+def _transpose_lol(lol):
+    """Transpose list of lists."""
+    return list(map(list, zip(*lol)))
diff --git a/xcp_d/workflows/concatenation.py b/xcp_d/workflows/concatenation.py
index 1d6157b8b..93c809bce 100644
--- a/xcp_d/workflows/concatenation.py
+++ b/xcp_d/workflows/concatenation.py
@@ -12,7 +12,7 @@
     FilterOutFailedRuns,
 )
 from xcp_d.utils.doc import fill_doc
-from xcp_d.utils.utils import _make_dictionary, _select_first
+from xcp_d.utils.utils import _make_dictionary, _select_first, _transpose_lol
 from xcp_d.workflows.plotting import init_qc_report_wf
 
 
@@ -311,7 +311,11 @@ def init_concatenate_data_wf(
         name="make_timeseries_dict",
         iterfield=["Sources"],
     )
-    workflow.connect([(timeseries_src, make_timeseries_dict, [("bids_uris", "Sources")])])
+    # fmt:off
+    workflow.connect([
+        (timeseries_src, make_timeseries_dict, [(("bids_uris", _transpose_lol), "Sources")]),
+    ])
+    # fmt:on
 
     ds_timeseries = pe.MapNode(
         DerivativesDataSink(