diff --git a/app/assets/javascripts/discourse/components/group-membership-button.js.es6 b/app/assets/javascripts/discourse/components/group-membership-button.js.es6
index 8f1ac5798a7..b78bb902064 100644
--- a/app/assets/javascripts/discourse/components/group-membership-button.js.es6
+++ b/app/assets/javascripts/discourse/components/group-membership-button.js.es6
@@ -17,7 +17,7 @@ export default Ember.Component.extend({
     if (isGroupUser) {
       return isGroupUser;
     } else {
-      return groupUserIds.includes(groupId);
+      return !!groupUserIds && groupUserIds.includes(groupId);
     }
   },
 
diff --git a/test/javascripts/components/group-membership-button-test.js.es6 b/test/javascripts/components/group-membership-button-test.js.es6
index 579aa6f1adf..b1796078d87 100644
--- a/test/javascripts/components/group-membership-button-test.js.es6
+++ b/test/javascripts/components/group-membership-button-test.js.es6
@@ -35,3 +35,27 @@ test('canRequestMembership', function() {
 
   equal(this.subject().get('canRequestMembership'), false);
 });
+
+test('userIsGroupUser', function() {
+  this.subject().setProperties({
+    model: { is_group_user: true }
+  });
+
+  equal(this.subject().get('userIsGroupUser'), true);
+
+  this.subject().set('model.is_group_user', false);
+
+  equal(this.subject().get('userIsGroupUser'), false);
+
+  this.subject().setProperties({ model: { id: 1 }, groupUserIds: [1] });
+
+  equal(this.subject().get('userIsGroupUser'), true);
+
+  this.subject().set('groupUserIds', [3]);
+
+  equal(this.subject().get('userIsGroupUser'), false);
+
+  this.subject().set('groupUserIds', undefined);
+
+  equal(this.subject().get('userIsGroupUser'), false);
+});