diff --git a/ax/modelbridge/transforms/one_hot.py b/ax/modelbridge/transforms/one_hot.py
index f16d151519..b5d549b298 100644
--- a/ax/modelbridge/transforms/one_hot.py
+++ b/ax/modelbridge/transforms/one_hot.py
@@ -151,6 +151,8 @@ def untransform_observation_features(
                 val = self.encoder[p_name].inverse_transform(encoded_labels=x[None, :])[
                     0
                 ]
+                if isinstance(val, np.str_):
+                    val = str(val)
                 if isinstance(val, np.bool_):
                     val = bool(val)  # Numpy bools don't serialize
                 obsf.parameters[p_name] = val