diff --git a/src/wp-includes/class-wp-user-query.php b/src/wp-includes/class-wp-user-query.php
index 21d013ca28..1bc08aa491 100644
--- a/src/wp-includes/class-wp-user-query.php
+++ b/src/wp-includes/class-wp-user-query.php
@@ -75,6 +75,40 @@ class WP_User_Query {
 		}
 	}
 
+	/**
+	 * Fills in missing query variables with default values.
+	 *
+	 * @since 4.4.0
+	 * @access public
+	 *
+	 * @param array $args Query vars, as passed to `WP_User_Query`.
+	 * @return array Complete query variables with undefined ones filled in with defaults.
+	 */
+	public static function fill_query_vars( $args ) {
+		$defaults = array(
+			'blog_id' => $GLOBALS['blog_id'],
+			'role' => '',
+			'meta_key' => '',
+			'meta_value' => '',
+			'meta_compare' => '',
+			'include' => array(),
+			'exclude' => array(),
+			'search' => '',
+			'search_columns' => array(),
+			'orderby' => 'login',
+			'order' => 'ASC',
+			'offset' => '',
+			'number' => '',
+			'paged' => 1,
+			'count_total' => true,
+			'fields' => 'all',
+			'who' => '',
+			'has_published_posts' => null,
+		);
+
+		return wp_parse_args( $args, $defaults );
+	}
+
 	/**
 	 * Prepare the query variables.
 	 *
@@ -146,26 +180,7 @@ class WP_User_Query {
 
 		if ( empty( $this->query_vars ) || ! empty( $query ) ) {
 			$this->query_limit = null;
-			$this->query_vars = wp_parse_args( $query, array(
-				'blog_id' => $GLOBALS['blog_id'],
-				'role' => '',
-				'meta_key' => '',
-				'meta_value' => '',
-				'meta_compare' => '',
-				'include' => array(),
-				'exclude' => array(),
-				'search' => '',
-				'search_columns' => array(),
-				'orderby' => 'login',
-				'order' => 'ASC',
-				'offset' => '',
-				'number' => '',
-				'paged' => 1,
-				'count_total' => true,
-				'fields' => 'all',
-				'who' => '',
-				'has_published_posts' => null,
-			) );
+			$this->query_vars = $this->fill_query_vars( $query );
 		}
 
 		/**
@@ -181,7 +196,9 @@ class WP_User_Query {
 		 */
 		do_action( 'pre_get_users', $this );
 
+		// Ensure that query vars are filled after 'pre_get_users'.
 		$qv =& $this->query_vars;
+		$qv =  $this->fill_query_vars( $qv );
 
 		if ( is_array( $qv['fields'] ) ) {
 			$qv['fields'] = array_unique( $qv['fields'] );
diff --git a/tests/phpunit/tests/user/query.php b/tests/phpunit/tests/user/query.php
index 6bb95d5730..064c73f1ff 100644
--- a/tests/phpunit/tests/user/query.php
+++ b/tests/phpunit/tests/user/query.php
@@ -874,4 +874,26 @@ class Tests_User_Query extends WP_UnitTestCase {
 
 		$this->assertEquals( array( $users[2], $users[1] ), $q->results );
 	}
+
+	/**
+	 * @ticket 33449
+	 */
+	public function test_query_vars_should_be_filled_in_after_pre_get_users() {
+		$query_vars = array( 'blog_id', 'role', 'meta_key', 'meta_value', 'meta_compare', 'include', 'exclude', 'search', 'search_columns', 'orderby', 'order', 'offset', 'number', 'paged', 'count_total', 'fields', 'who', 'has_published_posts' );
+
+		add_action( 'pre_get_users', array( $this, 'filter_pre_get_users_args' ) );
+		$q = new WP_User_Query( array_fill_keys( $query_vars, '1' ) );
+		remove_action( 'pre_get_users', array( $this, 'filter_pre_get_users_args' ) );
+
+		foreach ( $query_vars as $query_var ) {
+			$this->assertTrue( array_key_exists( $query_var, $q->query_vars ), "$query_var does not exist." );
+		}
+
+	}
+
+	public function filter_pre_get_users_args( $q ) {
+		foreach ( $q->query_vars as $k => $v ) {
+			unset( $q->query_vars[ $k ] );
+		}
+	}
 }