diff --git a/smash/web/forms/study_subject_forms.py b/smash/web/forms/study_subject_forms.py
index bc97e8b069bc1abefce9d7628d9384392f8c5995..208410ad403725a147ea1e63a133fb3c5e57bab6 100644
--- a/smash/web/forms/study_subject_forms.py
+++ b/smash/web/forms/study_subject_forms.py
@@ -192,7 +192,9 @@ def validate_subject_screening_number(self, cleaned_data):
 def validate_subject_nd_number(self, cleaned_data):
     if self.study.columns.nd_number:
         nd_number = cleaned_data['nd_number']
-        if nd_number != "":
+        if nd_number is None:
+            self.add_error('nd_number', "None ND number. ND number can be blank but not None.")
+        elif nd_number:
             if not self.study.check_nd_number(nd_number):
                 self.add_error('nd_number', "Invalid ND number")
             else:
@@ -201,7 +203,7 @@ def validate_subject_nd_number(self, cleaned_data):
                 if subjects_from_db:
                     if subjects_from_db[0].screening_number != cleaned_data.get('screening_number', ''):
                         self.add_error('nd_number', "ND number already in use")
-
+        #else: #empty nd_number is valid
 
 def validate_subject_resign_reason(self, cleaned_data):
     if self.study.columns.resigned and self.study.columns.resign_reason: