diff --git a/sklearn2pmml/decoration/__init__.py b/sklearn2pmml/decoration/__init__.py
index e593e81..b77eab6 100644
--- a/sklearn2pmml/decoration/__init__.py
+++ b/sklearn2pmml/decoration/__init__.py
@@ -1,4 +1,5 @@
 from pandas import DataFrame
+from pandas.api.types import is_object_dtype
 from sklearn.base import clone, BaseEstimator, TransformerMixin
 from sklearn2pmml import _is_pandas_categorical
 from sklearn2pmml.util import cast, common_dtype, is_1d, to_numpy
@@ -112,6 +113,7 @@ def is_missing(X, missing_value):
 				if isinstance(missing_value, float) and numpy.isnan(missing_value):
 					return pandas.isnull(X)
 				return X == missing_value
+
 			if type(self.missing_values) is list:
 				mask = numpy.full(X.shape, fill_value = False)
 				for missing_value in self.missing_values:
@@ -129,8 +131,14 @@ def _transform_missing_values(self, X, where):
 		if self.missing_value_treatment == "return_invalid":
 			if numpy.any(where) > 0:
 				raise ValueError("Data contains {0} missing values".format(numpy.count_nonzero(where)))
-		if self.missing_value_replacement is not None:
-			X[where] = self.missing_value_replacement
+		elif self.missing_value_treatment in ["as_is", "as_mean", "as_mode", "as_median", "as_value"]:
+			if self.missing_value_replacement is not None:
+				X[where] = self.missing_value_replacement
+			# Special case for object data type columns: replacing non-None values with None values
+			elif is_object_dtype(self.dtype_) and (self.missing_value_replacement is None):
+				X[where] = self.missing_value_replacement
+		else:
+			raise ValueError()
 
 	def _transform_valid_values(self, X, where):
 		pass
@@ -146,6 +154,8 @@ def _transform_invalid_values(self, X, where):
 		elif self.invalid_value_treatment == "as_value":
 			if self.invalid_value_replacement is not None:
 				X[where] = self.invalid_value_replacement
+		else:
+			raise ValueError()
 
 	def _compute_masks(self, X):
 		X = to_numpy(X)
diff --git a/sklearn2pmml/decoration/tests/__init__.py b/sklearn2pmml/decoration/tests/__init__.py
index 7706ee6..333b212 100644
--- a/sklearn2pmml/decoration/tests/__init__.py
+++ b/sklearn2pmml/decoration/tests/__init__.py
@@ -145,7 +145,7 @@ def test_fit_int64(self):
 		self.assertEqual([False, True, False, False, False], domain._missing_value_mask(X).tolist())
 		Xt = domain.fit_transform(X)
 		self.assertEqual([-1, 1, 2], domain.data_values_.tolist())
-		self.assertEqual([-1, pandas.NA, 1, 2, -1], Xt.tolist())
+		self.assertEqual([-1, None, 1, 2, -1], Xt.tolist())
 
 	def test_fit_string(self):
 		domain = clone(CategoricalDomain(with_data = False, with_statistics = False))
@@ -200,11 +200,11 @@ def test_fit_string_missing(self):
 		self.assertEqual(["one", "two", "0"], Xt[:, 1].tolist())
 
 	def test_fit_string_valid(self):
-		domain = clone(CategoricalDomain(data_values = [["1", "2", "3"], ["zero", "one", "two"]], invalid_value_treatment = "as_value", invalid_value_replacement = "X"))
+		domain = clone(CategoricalDomain(data_values = [["1", "2", "3"], ["zero", "one", "two"]], invalid_value_treatment = "as_missing"))
 		self.assertEqual("as_is", domain.missing_value_treatment)
 		self.assertIsNone(domain.missing_value_replacement)
-		self.assertEqual("as_value", domain.invalid_value_treatment)
-		self.assertEqual("X", domain.invalid_value_replacement)
+		self.assertEqual("as_missing", domain.invalid_value_treatment)
+		self.assertIsNone(domain.invalid_value_replacement)
 		self.assertTrue(hasattr(domain, "data_values"))
 		self.assertFalse(hasattr(domain, "data_values_"))
 		X = DataFrame([["-1", None], ["0", "zero"], ["3", None], ["1", "two"], ["2", "one"], ["0", "three"]])
@@ -212,8 +212,8 @@ def test_fit_string_valid(self):
 		self.assertIsInstance(Xt, DataFrame)
 		self.assertTrue(hasattr(domain, "data_values"))
 		self.assertTrue(hasattr(domain, "data_values_"))
-		self.assertEqual(["X", "X", "3", "1", "2", "X"], Xt.iloc[:, 0].tolist())
-		self.assertEqual([None, "zero", None, "two", "one", "X"], Xt[1].tolist())
+		self.assertEqual([None, None, "3", "1", "2", None], Xt.iloc[:, 0].tolist())
+		self.assertEqual([None, "zero", None, "two", "one", None], Xt[1].tolist())
 		self.assertEqual(2, len(domain.counts_))
 		self.assertEqual({"totalFreq" : 6, "missingFreq" : 0, "invalidFreq" : 3}, domain.counts_[0])
 		self.assertEqual({"totalFreq" : 6, "missingFreq" : 2, "invalidFreq" : 1}, domain.counts_[1])