diff --git a/gramps/gui/views/treemodels/treebasemodel.py b/gramps/gui/views/treemodels/treebasemodel.py
index 2d68d1859..5739f4f1d 100644
--- a/gramps/gui/views/treemodels/treebasemodel.py
+++ b/gramps/gui/views/treemodels/treebasemodel.py
@@ -976,7 +976,7 @@ class TreeBaseModel(GObject.GObject, Gtk.TreeModel):
         if not secondary:
             # None is used to indicate this column has no data
             if self.fmap[col] is None:
-                return None
+                return ''
             value = self.fmap[col](data)
         else:
             if self.fmap2[col] is None: