diff --git a/sdcflows/workflows/tests/test_base.py b/sdcflows/workflows/tests/test_base.py
index f9f48a5440..234c89058e 100644
--- a/sdcflows/workflows/tests/test_base.py
+++ b/sdcflows/workflows/tests/test_base.py
@@ -63,4 +63,12 @@ def test_fmap_wf(tmpdir, workdir, outdir, bids_layouts, dataset, subject):
         wf.base_dir = str(workdir)
 
     if os.getenv("GITHUB_ACTIONS") != "true":
-        wf.run(plugin="Linear")
+        res = wf.run(plugin="Linear")
+
+    # Regression test for when out_merge_fmap_coeff was flattened and would
+    # have twice as many elements as the other nodes
+    assert all(
+        len(node.result.outputs.out) == len(estimators)
+        for node in res.nodes
+        if node.name.startswith("out_merge_")
+    )