Modify _values_for_factorize in sparse.py

Author: makbigc

pandas/core/arrays/sparse.py

@@ -942,7 +942,7 @@ def unique(self):
 
     def _values_for_factorize(self):
         # Still override this for hash_pandas_object
-        return np.asarray(self), self.fill_value
+        return np.asarray(self, object), self.fill_value
 
     def factorize(self, na_sentinel=-1):
         # Currently, ExtensionArray.factorize -> Tuple[ndarray, EA]