diff --git a/openmmtools/multistate/multistatesampler.py b/openmmtools/multistate/multistatesampler.py
index c32129be..31d49ffa 100644
--- a/openmmtools/multistate/multistatesampler.py
+++ b/openmmtools/multistate/multistatesampler.py
@@ -1025,12 +1025,12 @@ def _read_options(check_iteration):
         self._thermodynamic_states = thermodynamic_states
         self._unsampled_states = unsampled_states
         self._sampler_states = sampler_states
-        self._replica_thermodynamic_states = state_indices
+        self._replica_thermodynamic_states = np.array(state_indices)
         self._energy_thermodynamic_states = energy_thermodynamic_states
         self._neighborhoods = neighborhoods
         self._energy_unsampled_states = energy_unsampled_states
-        self._n_accepted_matrix = n_accepted_matrix
-        self._n_proposed_matrix = n_proposed_matrix
+        self._n_accepted_matrix = np.array(n_accepted_matrix)
+        self._n_proposed_matrix = np.array(n_proposed_matrix)
         self._metadata = metadata
 
         self._last_mbar_f_k = last_mbar_f_k