diff --git a/qiskit_dynamics/perturbation/solve_lmde_perturbation.py b/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
index 8d62050fc..4441865dd 100644
--- a/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
+++ b/qiskit_dynamics/perturbation/solve_lmde_perturbation.py
@@ -230,8 +230,9 @@ def solve_lmde_perturbation(
         else:
             # validate perturbation_labels
             perturbations_len = len(perturbation_labels)
-            perturbation_labels_reduced = _clean_multisets(perturbation_labels)
-            if len(perturbation_labels_reduced) != perturbations_len:
+            perturbation_labels = [Multiset(x) for x in perturbation_labels]
+            cleaned_perturbation_labels = _clean_multisets(perturbation_labels)
+            if len(cleaned_perturbation_labels) != perturbations_len:
                 raise QiskitError("perturbation_labels argument contains duplicates as multisets.")
 
         expansion_labels = _merge_multiset_expansion_order_labels(