diff --git a/modules/redirect_domain/src/Form/RedirectDomainForm.php b/modules/redirect_domain/src/Form/RedirectDomainForm.php
index 5c40b68..ec7b20f 100644
--- a/modules/redirect_domain/src/Form/RedirectDomainForm.php
+++ b/modules/redirect_domain/src/Form/RedirectDomainForm.php
@@ -152,7 +152,7 @@ class RedirectDomainForm extends ConfigFormBase {
           // Replace '.' with ':' for an eligible key.
           $redirect['from'] = str_replace('.', ':', $redirect['from']);
           $domain_redirects[$redirect['from']][] = [
-            'sub_path' => ltrim($redirect['sub_path'], '/') . '/',
+            'sub_path' => '/' . ltrim($redirect['sub_path'], '/'),
             'destination' => $redirect['destination']
           ];
         }
diff --git a/modules/redirect_domain/src/Tests/RedirectDomainUITest.php b/modules/redirect_domain/src/Tests/RedirectDomainUITest.php
index a8bd463..d50791f 100644
--- a/modules/redirect_domain/src/Tests/RedirectDomainUITest.php
+++ b/modules/redirect_domain/src/Tests/RedirectDomainUITest.php
@@ -40,9 +40,10 @@ class RedirectDomainUITest extends WebTestBase {
     // Add another field for new domain redirect.
     $this->drupalPostAjaxForm(NULL, [], ['op' => t('Add another')]);
 
-    // Add two new domain redirects, one without sub path.
+    // Add two new domain redirects.
     $edit = [
       'redirects[0][from]' => 'foo.example.org',
+      'redirects[0][sub_path]' => '//sub-path',
       'redirects[0][destination]' => 'www.example.org/foo',
       'redirects[1][from]' => 'bar.example.org',
       'redirects[1][sub_path]' => '',
@@ -57,7 +58,7 @@ class RedirectDomainUITest extends WebTestBase {
     $this->assertFieldByName('redirects[1][destination]', 'www.example.org/bar');
 
     // Ensure that the sub paths are correct.
-    $this->assertFieldByName('redirects[0][sub_path]', '/');
+    $this->assertFieldByName('redirects[0][sub_path]', '/sub-path');
     $this->assertFieldByName('redirects[1][sub_path]', '/');
   }
 }