diff --git a/trytond/ir/sequence.py b/trytond/ir/sequence.py
index fbc59cab30..b70dd89ab0 100644
--- a/trytond/ir/sequence.py
+++ b/trytond/ir/sequence.py
@@ -1,5 +1,6 @@
 import time
 from trytond.osv import fields, OSV, ExceptORM
+from string import Template
 
 
 class SequenceType(OSV):
@@ -26,6 +27,13 @@ class Sequence(OSV):
     number_increment = fields.Integer('Increment Number', required=True)
     padding = fields.Integer('Number padding', required=True)
 
+    def __init__(self):
+        super(Sequence, self).__init__()
+        self._constraints += [
+            ('check_prefix_suffix', 'Invalid prefix/suffix',
+                ['prefix', 'suffix']),
+        ]
+
     def default_active(self, cursor, user, context=None):
         return 1
 
@@ -42,12 +50,23 @@ class Sequence(OSV):
         cursor.execute('select code, name from ir_sequence_type')
         return cursor.fetchall()
 
+    def check_prefix_suffix(self, cursor, user, ids):
+        "Check prefix and suffix"
+
+        for sequence in self.browse(cursor, user, ids):
+            try:
+                self._process(sequence.prefix)
+                self._process(sequence.suffix)
+            except:
+                return False
+        return True
+
     def _process(self, string):
-        return (string or '') % {
-                'year':time.strftime('%Y'),
-                'month': time.strftime('%m'),
-                'day':time.strftime('%d'),
-                }
+        return Template(string or '').substitute(
+                year=time.strftime('%Y'),
+                month=time.strftime('%m'),
+                day=time.strftime('%d'),
+                )
 
     def get_id(self, cursor, user, sequence_id, test='id=%s'):
         cursor.execute('lock table ir_sequence')
diff --git a/trytond/ir/sequence.xml b/trytond/ir/sequence.xml
index 7736edd89b..eafb8d5676 100644
--- a/trytond/ir/sequence.xml
+++ b/trytond/ir/sequence.xml
@@ -25,11 +25,11 @@
                     <field name="number_next"/>
                     <separator string="Legend (for prefix, suffix)" colspan="4"/>
                     <label string="Year:"/>
-                    <label string="%%(year)s" colspan="3" align="0.0"/>
+                    <label string="${year}" colspan="3" align="0.0"/>
                     <label string="Month:"/>
-                    <label string="%%(month)s" colspan="3" align="0.0"/>
+                    <label string="${month}" colspan="3" align="0.0"/>
                     <label string="Day:"/>
-                    <label string="%%(day)s" colspan="3" align="0.0"/>
+                    <label string="${day}" colspan="3" align="0.0"/>
                 </form>
                 ]]>
             </field>