diff --git a/polygerrit-ui/app/elements/admin/gr-permission/gr-permission.js b/polygerrit-ui/app/elements/admin/gr-permission/gr-permission.js
index e719154..508c3a2 100644
--- a/polygerrit-ui/app/elements/admin/gr-permission/gr-permission.js
+++ b/polygerrit-ui/app/elements/admin/gr-permission/gr-permission.js
@@ -271,7 +271,12 @@
       // API to work as expected.
       const groupId = decodeURIComponent(e.detail.value.id)
           .replace(/\+/g, ' ');
-      this.set(['permission', 'value', 'rules', groupId], {});
+      // We cannot use "this.set(...)" here, because groupId may contain dots,
+      // and dots in property path names are totally unsupported by Polymer.
+      // Apparently Polymer picks up this change anyway, otherwise we should
+      // have looked at using MutableData:
+      // https://polymer-library.polymer-project.org/2.0/docs/devguide/data-system#mutable-data
+      this.permission.value.rules[groupId] = {};
 
       // Purposely don't recompute sorted array so that the newly added rule
       // is the last item of the array.
@@ -292,7 +297,8 @@
       Polymer.dom.flush();
       const value = this._rules[this._rules.length - 1].value;
       value.added = true;
-      this.set(['permission', 'value', 'rules', groupId], value);
+      // See comment above for why we cannot use "this.set(...)" here.
+      this.permission.value.rules[groupId] = value;
       this.dispatchEvent(
           new CustomEvent('access-modified', {bubbles: true, composed: true}));
     }
diff --git a/polygerrit-ui/app/elements/admin/gr-permission/gr-permission_test.html b/polygerrit-ui/app/elements/admin/gr-permission/gr-permission_test.html
index 7919b28..9928b9e 100644
--- a/polygerrit-ui/app/elements/admin/gr-permission/gr-permission_test.html
+++ b/polygerrit-ui/app/elements/admin/gr-permission/gr-permission_test.html
@@ -312,11 +312,11 @@
         element.name = 'Priority';
         element.section = 'refs/*';
         element.groups = {};
-        element.$.groupAutocomplete.text = 'ldap/tests tests';
+        element.$.groupAutocomplete.text = 'ldap/tests te.st';
         const e = {
           detail: {
             value: {
-              id: 'ldap:CN=test+test',
+              id: 'ldap:CN=test+te.st',
             },
           },
         };
@@ -325,11 +325,11 @@
         assert.equal(Object.keys(element._groupsWithRules).length, 2);
         element._handleAddRuleItem(e);
         flushAsynchronousOperations();
-        assert.deepEqual(element.groups, {'ldap:CN=test test': {
-          name: 'ldap/tests tests'}});
+        assert.deepEqual(element.groups, {'ldap:CN=test te.st': {
+          name: 'ldap/tests te.st'}});
         assert.equal(element._rules.length, 3);
         assert.equal(Object.keys(element._groupsWithRules).length, 3);
-        assert.deepEqual(element.permission.value.rules['ldap:CN=test test'],
+        assert.deepEqual(element.permission.value.rules['ldap:CN=test te.st'],
             {action: 'ALLOW', min: -2, max: 2, added: true});
         // New rule should be removed if cancel from editing.
         element.editing = false;