diff --git a/src/calibre/db/tables.py b/src/calibre/db/tables.py
index 3e1db9400f..9b9ff4e9e0 100644
--- a/src/calibre/db/tables.py
+++ b/src/calibre/db/tables.py
@@ -113,7 +113,7 @@ class SizeTable(OneToOneTable):
         for row in db.conn.execute(
                 'SELECT books.id, (SELECT MAX(uncompressed_size) FROM data '
                 'WHERE data.book=books.id) FROM books'):
-            self.book_col_map[row[0]] = self.unserialize(row[1] or 0)
+            self.book_col_map[row[0]] = self.unserialize(row[1])
 
     def update_sizes(self, size_map):
         self.book_col_map.update(size_map)
diff --git a/src/calibre/gui2/library/models.py b/src/calibre/gui2/library/models.py
index 4a1a1a3e88..5705f57000 100644
--- a/src/calibre/gui2/library/models.py
+++ b/src/calibre/gui2/library/models.py
@@ -660,7 +660,7 @@ class BooksModel(QAbstractTableModel):  # {{{
             elif field == 'size':
                 sz_mult = 1.0/(1024**2)
                 def func(idx):
-                    val = fffunc(field_obj, idfunc(idx), default_value=0)
+                    val = fffunc(field_obj, idfunc(idx), default_value=0) or 0
                     ans = u'%.1f' % (val * sz_mult)
                     if val > 0 and ans == u'0.0':
                         ans = u'<0.1'