diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 9effbadcd5..feec9b3de4 100644
--- a/src/wp-includes/theme.php
+++ b/src/wp-includes/theme.php
@@ -2607,16 +2607,19 @@ function add_theme_support( $feature, ...$args ) {
 
 		case 'html5':
 			// You can't just pass 'html5', you need to pass an array of types.
-			if ( empty( $args[0] ) ) {
-				// Build an array of types for back-compat.
-				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form' ) );
-			} elseif ( ! isset( $args[0] ) || ! is_array( $args[0] ) ) {
+			if ( empty( $args[0] ) || ! is_array( $args[0] ) ) {
 				_doing_it_wrong(
 					"add_theme_support( 'html5' )",
 					__( 'You need to pass an array of types.' ),
 					'3.6.1'
 				);
-				return false;
+
+				if ( ! empty( $args[0] ) && ! is_array( $args[0] ) ) {
+					return false;
+				}
+
+				// Build an array of types for back-compat.
+				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form' ) );
 			}
 
 			// Calling 'html5' again merges, rather than overwrites.
diff --git a/tests/phpunit/tests/theme/support.php b/tests/phpunit/tests/theme/support.php
index a6bdf5efac..aed9d94d3c 100644
--- a/tests/phpunit/tests/theme/support.php
+++ b/tests/phpunit/tests/theme/support.php
@@ -76,11 +76,18 @@ class Tests_Theme_Support extends WP_UnitTestCase {
 
 	/**
 	 * @ticket 24932
+	 *
+	 * @expectedIncorrectUsage add_theme_support( 'html5' )
 	 */
 	public function test_supports_html5() {
 		remove_theme_support( 'html5' );
 		$this->assertFalse( current_theme_supports( 'html5' ) );
 		$this->assertFalse( current_theme_supports( 'html5', 'comment-form' ) );
+
+		/*
+		 * If the second parameter is not specified, it should throw a _doing_it_wrong() notice
+		 * and fall back to `array( 'comment-list', 'comment-form', 'search-form' )` for back-compat.
+		 */
 		$this->assertNotFalse( add_theme_support( 'html5' ) );
 		$this->assertTrue( current_theme_supports( 'html5' ) );
 		$this->assertTrue( current_theme_supports( 'html5', 'comment-form' ) );
@@ -98,6 +105,8 @@ class Tests_Theme_Support extends WP_UnitTestCase {
 		remove_theme_support( 'html5' );
 		$this->assertFalse( current_theme_supports( 'html5' ) );
 		$this->assertFalse( current_theme_supports( 'html5', 'comment-form' ) );
+
+		// The second parameter should be an array.
 		$this->assertFalse( add_theme_support( 'html5', 'comment-form' ) );
 		$this->assertNotFalse( add_theme_support( 'html5', array( 'comment-form' ) ) );
 		$this->assertTrue( current_theme_supports( 'html5', 'comment-form' ) );