diff --git a/mesmerize_core/algorithms/mcorr.py b/mesmerize_core/algorithms/mcorr.py
index fbfd1fd..3bac29e 100644
--- a/mesmerize_core/algorithms/mcorr.py
+++ b/mesmerize_core/algorithms/mcorr.py
@@ -112,7 +112,7 @@ def run_algo(batch_path, uuid, data_path: str = None):
             y_shifts = mc.y_shifts_els
             shifts = [x_shifts, y_shifts]
             if hasattr(mc, 'z_shifts_els'):
-                shifts += mc.z_shifts_els
+                shifts.append(mc.z_shifts_els)
             shift_path = output_dir.joinpath(f"{uuid}_shifts.npy")
             np.save(str(shift_path), shifts)
         else: