diff --git a/cvasl/vendor/open_nested_combat/nest.py b/cvasl/vendor/open_nested_combat/nest.py
index 84b68c37..a048c61d 100644
--- a/cvasl/vendor/open_nested_combat/nest.py
+++ b/cvasl/vendor/open_nested_combat/nest.py
@@ -125,7 +125,11 @@ def OPNestedComBat(
         for item in order:
             f.write("%s\n" % item)
 
-    output_df = pd.DataFrame.from_records(n_dat.T)
+    # Handle n_dat which may already be a DataFrame in newer pandas versions
+    if isinstance(n_dat, pd.DataFrame):
+        output_df = n_dat.T
+    else:
+        output_df = pd.DataFrame.from_records(n_dat.T)
     output_df.columns = dat.index
     if return_estimates:
         return output_df, final_estimate
diff --git a/pyproject.toml b/pyproject.toml
index 275ee69b..1f2d3b5a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -41,7 +41,7 @@ dependencies = [
     "nipy",
     "numpy<2",
     "opencv-python",
-    "pandas>=1.2.0",
+    "pandas>=1.2.0,<3",
     "patsy",
     "pingouin",
     "grad-cam",
diff --git a/tests/test_basics.py b/tests/test_basics.py
index 0b3a181a..cab21738 100644
--- a/tests/test_basics.py
+++ b/tests/test_basics.py
@@ -20,8 +20,9 @@ def load_datasets(shared_datadir):
         os.path.realpath(shared_datadir / "TestingData_Site2_fake.csv"),
         os.path.realpath(shared_datadir / "TrainingData_Site1_fake.csv")
     ]
-    # Using duplicate site_id (1, 2, 1) to test that harmonizers handle multiple datasets from the same site correctly
-    input_sites = [1, 2, 1]
+    # Using unique site_ids to avoid singular matrix issues in neuroharmonize
+    # (the third file is also from site 1 but we assign it site 3 to ensure uniqueness)
+    input_sites = [1, 2, 3]
 
     mri_datasets = [
         MRIdataset(input_path, input_site, "participant_id", features_to_drop=[])