diff --git a/src/Feed.php b/src/Feed.php
index 6fa38e8..d156b0d 100644
--- a/src/Feed.php
+++ b/src/Feed.php
@@ -138,18 +138,18 @@ public function getEntries(): array
 
     /**
      * @param string        $ns
-     * @param string        $url
+     * @param string        $uri
      * @param string        $name
      * @param mixed         $value
      * @param null|string[] $attributes
      */
-    public function addCustomElement(string $ns, string $url, string $name, $value, ?array $attributes = null): void
+    public function addCustomElement(string $ns, string $uri, string $name, $value, ?array $attributes = null): void
     {
-        self::assertURL($url);
+        self::assertURL($uri);
 
         $this->customElements[] = [
             'ns' => $ns,
-            'url' => $url,
+            'uri' => $uri,
             'name' => $name,
             'value' => $value,
             'attributes' => $attributes ?? [],
@@ -186,7 +186,7 @@ public function addChildrenTo(SimpleXMLElement $parent): void
         }
 
         foreach ($this->customElements as $customElement) {
-            $element = $parent->addChild($customElement['name'], $customElement['value'], $customElement['ns']);
+            $element = $parent->addChild($customElement['name'], $customElement['value'], $customElement['uri']);
             foreach ($customElement['attributes'] as $name => $value) {
                 $element->addAttribute($name, $value);
             }
@@ -207,7 +207,7 @@ public function getSimpleXML(): SimpleXMLElement
             $attributes['xml:lang'] = $this->language;
         }
         foreach ($this->customElements as $customElement) {
-            $attributes['xmlns:'.$customElement['ns']] = $customElement['url'];
+            $attributes['xmlns:'.$customElement['ns']] = $customElement['uri'];
         }
 
         $attributesString = '';
diff --git a/tests/FeedTest.php b/tests/FeedTest.php
index 423a32b..0c2e4d5 100644
--- a/tests/FeedTest.php
+++ b/tests/FeedTest.php
@@ -40,7 +40,7 @@ public function testFeedCreation1(): void
         $feed->addContributor('contributor', 'contributor@test.com', 'http://test.com/contributor');
         $feed->setUpdatedDateTime(new DateTime('2019-05-04T20:00:40Z'));
         $feed->addCustomElement('sy', 'http://purl.org/rss/1.0/modules/syndication', 'updatePeriod', 'hourly');
-        $feed->addCustomElement('sy', 'http://purl.org/rss/1.0/modules/syndication', 'sy:updateFrequency', 10);
+        $feed->addCustomElement('sy', 'http://purl.org/rss/1.0/modules/syndication', 'updateFrequency', 10);
 
         $entry = new Entry();
         $entry->setTitle('entry title', 'text');
diff --git a/tests/feed_1.xml b/tests/feed_1.xml
index 6f35558..b77fb6d 100644
--- a/tests/feed_1.xml
+++ b/tests/feed_1.xml
@@ -25,8 +25,8 @@
   <logo>http://test.com/logo</logo>
   <icon>http://test.com/icon</icon>
   <generator version="version" uri="http://test.com/generator">generator</generator>
-  <updatePeriod xmlns="sy">hourly</updatePeriod>
-  <sy:updateFrequency xmlns:sy="sy">10</sy:updateFrequency>
+  <sy:updatePeriod>hourly</sy:updatePeriod>
+  <sy:updateFrequency>10</sy:updateFrequency>
   <entry>
     <id>tag:entry-test</id>
     <title type="text">entry title</title>