diff --git a/smash/web/forms.py b/smash/web/forms.py
index 2294dd24246be9f0da6205e41a5cf3ad70e22317..80e00359544498c0bdcc726ca6e59067381e3958 100644
--- a/smash/web/forms.py
+++ b/smash/web/forms.py
@@ -88,7 +88,10 @@ def get_new_screening_number(screening_number_prefix):
             screening_number = screening_number.strip()
             if screening_number.startswith(screening_number_prefix):
                 number = screening_number[len(screening_number_prefix):]
-                result_number = max(result_number, int(number))
+                try:
+                    result_number = max(result_number, int(number))
+                except ValueError:
+                    pass
 
     return screening_number_prefix + str(result_number + 1).zfill(3)
 
diff --git a/smash/web/tests/test_SubjectAddForm.py b/smash/web/tests/test_SubjectAddForm.py
index de00468a4beda5930c56592887c3a87f516b9b5a..158814504a49e84a5c26e84e8e837ad9c5aa92d4 100644
--- a/smash/web/tests/test_SubjectAddForm.py
+++ b/smash/web/tests/test_SubjectAddForm.py
@@ -135,3 +135,12 @@ class SubjectAddFormTests(TestCase):
 
         new_screening_number = get_new_screening_number(prefix)
         self.assertEqual(prefix + "201", new_screening_number)
+
+    def test_get_new_screening_number_6(self):
+        prefix = "X-"
+        subject = create_subject()
+        subject.screening_number = "X-"
+        subject.save()
+
+        new_screening_number = get_new_screening_number(prefix)
+        self.assertEqual(prefix + "001", new_screening_number)