diff --git a/includes/Html/Html.php b/includes/Html/Html.php
index 4124d3f618b..98084cc1707 100644
--- a/includes/Html/Html.php
+++ b/includes/Html/Html.php
@@ -152,8 +152,9 @@ class Html {
 			if ( isset( $attrs['class'] ) ) {
 				$classAsArray = is_string( $attrs[ 'class' ] ) ? explode( ' ', $attrs[ 'class' ] ) : $attrs[ 'class' ];
 				if ( !in_array( $cdxInputClass, $classAsArray ) ) {
-					$attrs['class'][] = 'mw-ui-input';
+					$classAsArray[] = 'mw-ui-input';
 				}
+				$attrs['class'] = $classAsArray;
 			} else {
 				$attrs['class'] = 'mw-ui-input';
 			}
diff --git a/tests/phpunit/includes/HtmlTest.php b/tests/phpunit/includes/HtmlTest.php
index 7cd65e822db..4fa5d1273fc 100644
--- a/tests/phpunit/includes/HtmlTest.php
+++ b/tests/phpunit/includes/HtmlTest.php
@@ -11,7 +11,6 @@ class HtmlTest extends MediaWikiIntegrationTestCase {
 	protected function setUp(): void {
 		parent::setUp();
 
-		$this->overrideConfigValue( MainConfigNames::UseMediaWikiUIEverywhere, false );
 		$this->overrideConfigValue( MainConfigNames::UsePigLatinVariant, false );
 
 		$langFactory = $this->getServiceContainer()->getLanguageFactory();
@@ -831,6 +830,23 @@ class HtmlTest extends MediaWikiIntegrationTestCase {
 		}
 		$this->assertSame( $expected, $html );
 	}
+
+	public function testGetTextInputAttributes() {
+		$this->setMwGlobals( 'wgUseMediaWikiUIEverywhere', true );
+		$attrs = Html::getTextInputAttributes( [ 'class' => 'foo' ] );
+		$attrsNew = Html::getTextInputAttributes( [ 'class' => 'cdx-text-input__input' ] );
+		$attrsArray = Html::getTextInputAttributes( [
+			'class' => [ 'foo' ]
+		] );
+		$attrsArrayNew = Html::getTextInputAttributes( [
+			'class' => [ 'cdx-text-input__input', 'bar' ]
+		] );
+
+		$this->assertSame( [ 'foo', 'mw-ui-input' ], $attrs[ 'class' ] );
+		$this->assertSame( [ 'cdx-text-input__input' ], $attrsNew[ 'class' ] );
+		$this->assertSame( [ 'foo', 'mw-ui-input' ], $attrsArray[ 'class' ] );
+		$this->assertSame( [ 'cdx-text-input__input', 'bar' ], $attrsArrayNew[ 'class' ] );
+	}
 }
 
 class HtmlTestValue {