diff --git a/psipy/core/io/saveable_sklearn.py b/psipy/core/io/saveable_sklearn.py
index 1bde6b0..49dbbf1 100644
--- a/psipy/core/io/saveable_sklearn.py
+++ b/psipy/core/io/saveable_sklearn.py
@@ -13,6 +13,7 @@
     SaveableSklearnMixin
 
 """
+import numpy as np
 
 from typing import Any, Dict
 
@@ -53,7 +54,12 @@ def get_config(self) -> Dict[str, Any]:
 
     def _save(self, zipfile: MemoryZipFile) -> MemoryZipFile:
         zipfile.add("config.json", self.get_config())
-        zipfile.add_mixed_dict("state", self.__getstate__())
+        state_dict = self.__getstate__()
+        for key, arr in state_dict.items():
+            if isinstance(arr, np.ndarray) and arr.dtype == object:
+                if all(isinstance(x, str) for x in arr.flat):
+                    state_dict[key] = arr.astype(str)
+        zipfile.add_mixed_dict("state", state_dict)
         return zipfile
 
     @classmethod