diff --git a/src/calibre/gui2/custom_column_widgets.py b/src/calibre/gui2/custom_column_widgets.py
index 1758116e7a..3be4c19d17 100644
--- a/src/calibre/gui2/custom_column_widgets.py
+++ b/src/calibre/gui2/custom_column_widgets.py
@@ -299,7 +299,9 @@ class Series(Base):
         val, s_index = self.gui_val
         val = self.normalize_ui_val(val)
         if val != self.initial_val or s_index != self.initial_index:
-            if s_index == 0.0:
+            if val == '':
+                val = s_index = None
+            elif s_index == 0.0:
                 if tweaks['series_index_auto_increment'] == 'next':
                     s_index = self.db.get_next_cc_series_num_for(val,
                                                              num=self.col_id)
@@ -488,7 +490,7 @@ class BulkSeries(BulkBase):
 
     def commit(self, book_ids, notify=False):
         val, update_indices, force_start, at_value, clear = self.gui_val
-        val = '' if clear else self.normalize_ui_val(val)
+        val = None if clear else self.normalize_ui_val(val)
         if clear or val != '':
             extras = []
             next_index = self.db.get_next_cc_series_num_for(val, num=self.col_id)
diff --git a/src/calibre/gui2/library/models.py b/src/calibre/gui2/library/models.py
index 848c93f485..53e0982211 100644
--- a/src/calibre/gui2/library/models.py
+++ b/src/calibre/gui2/library/models.py
@@ -743,6 +743,8 @@ class BooksModel(QAbstractTableModel): # {{{
                 val = qt_to_dt(val, as_utc=False)
         elif typ == 'series':
             val, s_index = parse_series_string(self.db, label, value.toString())
+            if not val:
+                val = s_index = None
         elif typ == 'composite':
             tmpl = unicode(value.toString()).strip()
             disp = cc['display']