diff --git a/tests/phpunit/tests/theme.php b/tests/phpunit/tests/theme.php
index e3b16212650de..23a61eca2b6eb 100644
--- a/tests/phpunit/tests/theme.php
+++ b/tests/phpunit/tests/theme.php
@@ -94,8 +94,8 @@ public function test_get_theme() {
 
 		foreach ( array_keys( $themes ) as $name ) {
 			$theme = get_theme( $name );
-			// WP_Theme implements ArrayAccess. Even ArrayObject returns false for is_array().
-			$this->assertFalse( is_array( $theme ) );
+			// WP_Theme implements ArrayAccess, but that does not make it an array.
+			$this->assertIsNotArray( $theme );
 			$this->assertInstanceOf( 'WP_Theme', $theme );
 			$this->assertSame( $theme, $themes[ $name ] );
 		}