diff --git a/src/common.js b/src/common.js
index b16f923579..9aa0ede752 100644
--- a/src/common.js
+++ b/src/common.js
@@ -1183,17 +1183,8 @@
                     if (~['many2one', 'one2one', 'one2many', 'many2many']
                         .indexOf(field.type)) {
                         var test = function(value) {
-                            if (field.type == 'many2one') {
-                                if ((typeof value != 'string') &&
-                                    (value !== null)) {
-                                    return false;
-                                }
-                            } else {
-                                if (typeof value != 'string') {
-                                    return false;
-                                }
-                            }
-                            return true;
+                            return ((typeof value == 'string') ||
+                                (value === null));
                         };
                         if (value instanceof Array) {
                             return value.every(test);
diff --git a/tests/sao.js b/tests/sao.js
index 5c4e8049ad..f4b982671f 100644
--- a/tests/sao.js
+++ b/tests/sao.js
@@ -2109,6 +2109,7 @@
         QUnit.ok(parser.stringable([['relation.rec_name', '=', 'Foo']]));
         QUnit.ok(!parser.stringable([['relation', '=', 1]]));
         QUnit.ok(parser.stringable([['relations', '=', 'Foo']]));
+        QUnit.ok(parser.stringable([['relations', '=', null]]));
         QUnit.ok(parser.stringable([['relations', 'in', ['Foo']]]));
         QUnit.ok(!parser.stringable([['relations', 'in', [42]]]));
         QUnit.ok(parser.stringable([['relation.name', '=', "Foo"]]));