diff --git a/sklearn2pmml/preprocessing/__init__.py b/sklearn2pmml/preprocessing/__init__.py
index 817750c..e588a53 100644
--- a/sklearn2pmml/preprocessing/__init__.py
+++ b/sklearn2pmml/preprocessing/__init__.py
@@ -324,14 +324,14 @@ def __init__(self, mapping, default_value):
 		k_type = None
 		v_type = None
 		for k, v in mapping.items():
-			if k is None:
-				raise ValueError("Key is None")
+			if pandas.isnull(k):
+				raise ValueError("Key is a missing value")
 			if k_type is None:
 				k_type = type(k)
 			else:
 				if type(k) != k_type:
 					raise TypeError("Key is not a {0}".format(k_type.__name__))
-			if v is None:
+			if pandas.isnull(v):
 				continue
 			if v_type is None:
 				v_type = type(v)
@@ -357,8 +357,13 @@ def fit(self, X, y = None):
 	def transform(self, X):
 		X = ensure_1d(X)
 		transform_dict = self._transform_dict()
-		func = lambda k: transform_dict[k]
-		Xt = eval_rows(X, func)
+
+		def _eval_row(x):
+			if pandas.isnull(x):
+				return x
+			return transform_dict[x]
+
+		Xt = eval_rows(X, _eval_row)
 		return _col2d(Xt)
 
 class FilterLookupTransformer(LookupTransformer):
@@ -375,8 +380,8 @@ class FilterLookupTransformer(LookupTransformer):
 	def __init__(self, mapping):
 		super(FilterLookupTransformer, self).__init__(mapping, default_value = None)
 		for k, v in mapping.items():
-			if v is None:
-				raise ValueError("Value is None")
+			if pandas.isnull(v):
+				raise ValueError("Value is a missing value")
 			if type(k) != type(v):
 				raise TypeError("Key and Value type mismatch")
 
@@ -416,10 +421,16 @@ def fit(self, X, y = None):
 
 	def transform(self, X):
 		transform_dict = self._transform_dict()
-		# See https://stackoverflow.com/a/3460747
-		# See https://stackoverflow.com/a/3338368
-		func = lambda k: transform_dict[tuple(k) if isinstance(k, Hashable) else tuple(numpy.squeeze(numpy.asarray(k)))]
-		Xt = eval_rows(X, func)
+
+		def _eval_row(x):
+			if (pandas.isnull(x)).any():
+				return None
+			# See https://stackoverflow.com/a/3460747
+			# See https://stackoverflow.com/a/3338368
+			x = x if isinstance(x, Hashable) else tuple(numpy.squeeze(numpy.asarray(x)))
+			return transform_dict[tuple(x)]
+
+		Xt = eval_rows(X, _eval_row)
 		return _col2d(Xt)
 
 def _make_index(values):
diff --git a/sklearn2pmml/preprocessing/tests/__init__.py b/sklearn2pmml/preprocessing/tests/__init__.py
index 0bf6619..77cc2b6 100644
--- a/sklearn2pmml/preprocessing/tests/__init__.py
+++ b/sklearn2pmml/preprocessing/tests/__init__.py
@@ -335,12 +335,14 @@ def test_transform_float(self):
 		with self.assertRaises(TypeError):
 			LookupTransformer(mapping, int(0))
 		transformer = LookupTransformer(mapping, float("NaN"))
-		X = numpy.array([[0.0], [90.0]])
-		self.assertEqual([[math.cos(0.0)], [math.cos(90.0)]], transformer.transform(X).tolist())
-		X = numpy.array([180.0])
-		self.assertTrue(math.isnan(transformer.transform(X)))
 		X = Series([0.0, 45.0, 90.0])
 		self.assertEqual([[math.cos(0.0)], [math.cos(45.0)], [math.cos(90.0)]], transformer.transform(X).tolist())
+		X = numpy.array([[0.0], [90.0]])
+		self.assertEqual([[math.cos(0.0)], [math.cos(90.0)]], transformer.transform(X).tolist())
+		X = numpy.array([float("NaN"), 180.0])
+		self.assertTrue(nan_eq([[float("NaN")], [float("NaN")]], transformer.transform(X)))
+		transformer = LookupTransformer(mapping, -999.0)
+		self.assertTrue(nan_eq([[float("NaN")], [-999.0]], transformer.transform(X)))
 
 	def test_transform_string(self):
 		mapping = {
@@ -353,10 +355,12 @@ def test_transform_string(self):
 			LookupTransformer(mapping, None)
 		mapping.pop(None)
 		transformer = LookupTransformer(mapping, None)
-		X = numpy.array([["zero"], ["one"]])
-		self.assertEqual([[None], ["ein"]], transformer.transform(X).tolist())
 		X = Series(["one", "two", "three"])
 		self.assertEqual([["ein"], ["zwei"], ["drei"]], transformer.transform(X).tolist())
+		X = numpy.array([[None], ["zero"]])
+		self.assertEqual([[None], [None]], transformer.transform(X).tolist())
+		transformer = LookupTransformer(mapping, "(other)")
+		self.assertEqual([[None], ["(other)"]], transformer.transform(X).tolist())
 
 class FilterLookupTransformerTest(TestCase):
 
@@ -413,8 +417,10 @@ def test_transform_object(self):
 		transformer = MultiLookupTransformer(mapping, None)
 		X = DataFrame([["one", None], ["one", True], [None, True], ["two", True], ["three", True]])
 		self.assertEqual([[None], ["ein"], [None], ["zwei"], ["drei"]], transformer.transform(X).tolist())
-		X = numpy.matrix([["one", True], ["one", None], ["two", True]], dtype = "O")
-		self.assertEqual([["ein"], [None], ["zwei"]], transformer.transform(X).tolist())
+		X = numpy.matrix([["one", True], ["one", None], ["one", False], ["two", True]], dtype = "O")
+		self.assertEqual([["ein"], [None], [None], ["zwei"]], transformer.transform(X).tolist())
+		transformer = MultiLookupTransformer(mapping, "(other)")
+		self.assertEqual([["ein"], [None], ["(other)"], ["zwei"]], transformer.transform(X).tolist())
 
 class PMMLLabelBinarizerTest(TestCase):