diff -r 5b4101d67bb7 trytond/model/match.py
--- a/trytond/trytond/model/match.py	Mon Oct 15 22:49:51 2018 +0200
+++ b/trytond/trytond/model/match.py	Sat Nov 03 18:21:44 2018 +0100
@@ -10,6 +10,8 @@
         and matching value as value'''
         for field, pattern_value in pattern.iteritems():
             value = getattr(self, field)
+            if self._fields[field]._type in ('char', 'text') and value == "":
+                value = None
             if not match_none and value is None:
                 continue
             if self._fields[field]._type == 'many2one':