diff --git a/modules/inbound_email/inbound_email.py b/modules/inbound_email/inbound_email.py
index 9ce3ee815d..c682b8dd27 100644
--- a/modules/inbound_email/inbound_email.py
+++ b/modules/inbound_email/inbound_email.py
@@ -14,7 +14,7 @@ from functools import partial
 from trytond.config import config
 from trytond.model import ModelSQL, ModelView, fields, sequence_ordered
 from trytond.pool import Pool
-from trytond.pyson import Eval
+from trytond.pyson import Bool, Eval
 from trytond.transaction import Transaction
 from trytond.url import http_host
 
@@ -131,7 +131,7 @@ class Email(ModelSQL, ModelView):
         cls._order = [('id', 'DESC')]
         cls._buttons.update(
             process={
-                'readonly': Eval('rule'),
+                'readonly': Bool(Eval('rule')),
                 'depends': ['rule'],
                 },
             )