diff --git a/Orange/data/tests/test_variable.py b/Orange/data/tests/test_variable.py
index 4021acafcd4..efb35106f51 100644
--- a/Orange/data/tests/test_variable.py
+++ b/Orange/data/tests/test_variable.py
@@ -634,6 +634,8 @@ def test_val(self):
         self.assertEqual(a.str_val(""), "?")
         self.assertEqual(a.str_val(Value(a, "")), "?")
         self.assertEqual(a.repr_val(Value(a, "foo")), '"foo"')
+        self.assertEqual(a.str_val(np.nan), "?")
+        self.assertEqual(a.str_val(None), "?")
 
 
 @variabletest(TimeVariable)
diff --git a/Orange/data/variable.py b/Orange/data/variable.py
index 100b34b7e9c..a82f73af5f6 100644
--- a/Orange/data/variable.py
+++ b/Orange/data/variable.py
@@ -10,6 +10,7 @@
 from pickle import PickleError
 
 import numpy as np
+import pandas
 import scipy.sparse as sp
 
 from Orange.data import _variable
@@ -905,6 +906,8 @@ def str_val(val):
             if not val.value:
                 return "?"
             val = val.value
+        if pandas.isnull(val):
+            return "?"
         return str(val)
 
     def repr_val(self, val):