diff --git a/odoo/osv/expression.py b/odoo/osv/expression.py
index c1363c4f06d..761b9bef4fe 100644
--- a/odoo/osv/expression.py
+++ b/odoo/osv/expression.py
@@ -988,7 +988,7 @@ class expression(object):
                     else:
                         subquery = 'SELECT "%s" FROM "%s" WHERE "%s" IN %%s' % (rel_id1, rel_table, rel_id2)
                         # avoid flattening of argument in to_sql()
-                        subquery = cr.mogrify(subquery, [tuple(ids2)])
+                        subquery = cr.mogrify(subquery, [tuple(ids2)]).decode('utf-8')
                         push(create_substitution_leaf(leaf, ('id', 'inselect', (subquery, [])), internal=True))
                 else:
                     call_null_m2m = True
@@ -1018,7 +1018,7 @@ class expression(object):
                             subop = 'not inselect' if operator in NEGATIVE_TERM_OPERATORS else 'inselect'
                             subquery = 'SELECT "%s" FROM "%s" WHERE "%s" IN %%s' % (rel_id1, rel_table, rel_id2)
                             # avoid flattening of argument in to_sql()
-                            subquery = cr.mogrify(subquery, [tuple(it for it in res_ids if it)])
+                            subquery = cr.mogrify(subquery, [tuple(it for it in res_ids if it)]).decode('utf-8')
                             push(create_substitution_leaf(leaf, ('id', subop, (subquery, [])), internal=True))
 
                     if call_null_m2m: