diff --git a/plone/formwidget/contenttree/source.py b/plone/formwidget/contenttree/source.py
index 893ca64..8a1cee5 100644
--- a/plone/formwidget/contenttree/source.py
+++ b/plone/formwidget/contenttree/source.py
@@ -180,7 +180,7 @@ class PathSource(object):
     # brain. These will get hidden in the display templates
     def _placeholderTerm(self, value):
         return SimpleTerm(str(value),
-                          token='#error-missing-' + value,
+                          token='#error-missing-' + str(value),
                           title=u"Hidden or missing item '%s'" % value)