diff --git a/src/calibre/gui2/book_details.py b/src/calibre/gui2/book_details.py
index 6bc8d878ad..3e57442591 100644
--- a/src/calibre/gui2/book_details.py
+++ b/src/calibre/gui2/book_details.py
@@ -104,9 +104,10 @@ def render_data(mi, use_roman_numbers=True, all_fields=False):
             field = 'title_sort'
         if all_fields:
             display = True
-        isnull = mi.is_null(field)
         if metadata['datatype'] == 'bool':
             isnull = mi.get(field) is None
+        else:
+            isnull = mi.is_null(field)
         if (not display or not metadata or isnull or field == 'comments'):
             continue
         name = metadata['name']