--- a/web/formfields.py	Wed Sep 16 11:23:51 2015 +0200
+++ b/web/formfields.py	Mon Sep 14 16:03:07 2015 +0200
@@ -66,6 +66,8 @@
 from warnings import warn
 from datetime import datetime, timedelta
 
+from six import string_types
+
 from logilab.mtconverter import xml_escape
 from logilab.common import nullobject
 from logilab.common.date import ustrftime
@@ -842,7 +844,7 @@
             self.widget.attrs.setdefault('size', self.default_text_input_size)
 
     def _ensure_correctly_typed(self, form, value):
-        if isinstance(value, basestring):
+        if isinstance(value, string_types):
             value = value.strip()
             if not value:
                 return None
@@ -924,7 +926,7 @@
         return self.format_single_value(req, 1.234)
 
     def _ensure_correctly_typed(self, form, value):
-        if isinstance(value, basestring):
+        if isinstance(value, string_types):
             value = value.strip()
             if not value:
                 return None
@@ -956,7 +958,7 @@
         return u'20s, 10min, 24h, 4d'
 
     def _ensure_correctly_typed(self, form, value):
-        if isinstance(value, basestring):
+        if isinstance(value, string_types):
             value = value.strip()
             if not value:
                 return None
@@ -986,7 +988,7 @@
         return self.format_single_value(req, datetime.now())
 
     def _ensure_correctly_typed(self, form, value):
-        if isinstance(value, basestring):
+        if isinstance(value, string_types):
             value = value.strip()
             if not value:
                 return None