diff --git a/addons/web/static/src/js/views/basic/basic_model.js b/addons/web/static/src/js/views/basic/basic_model.js
index 304f7e475f7..47d5cee5c9c 100644
--- a/addons/web/static/src/js/views/basic/basic_model.js
+++ b/addons/web/static/src/js/views/basic/basic_model.js
@@ -942,10 +942,16 @@ var BasicModel = AbstractModel.extend({
                                     params.res_id = command[1];
                                 }
                                 rec = self._makeDataPoint(params);
-                                var data = command[2];
-                                _.each(Object.keys(data), function (name) {
+                                var data = {};
+                                _.each(Object.keys(command[2]), function (name) {
                                     var field = rec.fields[name];
-                                    if (field && field.type === 'many2one') {
+                                    if (!field) {
+                                        // ignore if this field is unknown as we don't
+                                        // know if it requires a special handling
+                                        return;
+                                    }
+                                    data[name] = command[2][name];
+                                    if (field.type === 'many2one') {
                                         var r = self._makeDataPoint({
                                             modelName: field.relation,
                                             data: {
@@ -957,9 +963,6 @@ var BasicModel = AbstractModel.extend({
                                     }
                                 });
                                 rec._changes = data;
-                                for (var f in rec._changes) {
-                                    rec.data[f] = null;
-                                }
                                 list._changes.push(rec.id);
                             }
                             if (command[0] === 5) {
diff --git a/addons/web/static/test/views/form_tests.js b/addons/web/static/test/views/form_tests.js
index 06616a407d6..a202ba93995 100644
--- a/addons/web/static/test/views/form_tests.js
+++ b/addons/web/static/test/views/form_tests.js
@@ -2275,6 +2275,72 @@ QUnit.module('Views', {
         form.destroy();
     });
 
+    QUnit.test('onchanges on unknown fields of o2m are ignored', function (assert) {
+        // many2one fields need to be postprocessed (the onchange returns [id,
+        // display_name]), but if we don't know the field, we can't know it's a
+        // many2one, so it isn't ignored, its value is an array instead of a
+        // dataPoint id, which may cause errors later (e.g. when saving).
+        assert.expect(2);
+
+        this.data.partner.records[1].p = [4];
+        this.data.partner.onchanges = {
+            foo: function () {},
+        };
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form>' +
+                    '<group>' +
+                        '<field name="foo"/>' +
+                        '<field name="int_field"/>' +
+                        '<field name="p">' +
+                            '<tree>' +
+                                '<field name="foo"/>' +
+                                '<field name="bar"/>' +
+                            '</tree>' +
+                            '<form>' +
+                                '<field name="foo"/>' +
+                                '<field name="product_id"/>' +
+                            '</form>' +
+                        '</field>' +
+                    '</group>' +
+                '</form>',
+            res_id: 2,
+            mockRPC: function (route, args) {
+                if (args.method === 'onchange') {
+                    return $.when({
+                        value: {
+                            p: [
+                                [5],
+                                [1, 4, {
+                                    foo: 'foo changed',
+                                    product_id: [37, "xphone"],
+                                }]
+                            ],
+                        },
+                    });
+                }
+                if (args.method === 'write') {
+                    assert.deepEqual(args.args[1].p, [[1, 4, {
+                        foo: 'foo changed',
+                    }]], "should only write value of known fields");
+                }
+                return this._super.apply(this, arguments);
+            },
+        });
+
+        form.$buttons.find('.o_form_button_edit').click();
+        form.$('.o_form_input:first').val('trigger an onchange').trigger('input');
+
+        assert.strictEqual(form.$('.o_data_row td:first').text(), 'foo changed',
+            "onchange should have been correctly applied on field in o2m list");
+
+        form.$buttons.find('.o_form_button_save').click();
+
+        form.destroy();
+    });
+
     QUnit.test('navigation with tab key in form view', function (assert) {
         assert.expect(2);