Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 25381)
+++ src/wp-includes/user.php	(working copy)
@@ -409,39 +409,44 @@
 			$this->query_fields = "$wpdb->users.ID";
 		}
 
-		if ( $qv['count_total'] )
+		if ( isset( $qv['count_total'] ) && $qv['count_total'] )
 			$this->query_fields = 'SQL_CALC_FOUND_ROWS ' . $this->query_fields;
 
 		$this->query_from = "FROM $wpdb->users";
 		$this->query_where = "WHERE 1=1";
 
 		// sorting
-		if ( in_array( $qv['orderby'], array('nicename', 'email', 'url', 'registered') ) ) {
-			$orderby = 'user_' . $qv['orderby'];
-		} elseif ( in_array( $qv['orderby'], array('user_nicename', 'user_email', 'user_url', 'user_registered') ) ) {
-			$orderby = $qv['orderby'];
-		} elseif ( 'name' == $qv['orderby'] || 'display_name' == $qv['orderby'] ) {
-			$orderby = 'display_name';
-		} elseif ( 'post_count' == $qv['orderby'] ) {
-			// todo: avoid the JOIN
-			$where = get_posts_by_author_sql('post');
-			$this->query_from .= " LEFT OUTER JOIN (
-				SELECT post_author, COUNT(*) as post_count
-				FROM $wpdb->posts
-				$where
-				GROUP BY post_author
-			) p ON ({$wpdb->users}.ID = p.post_author)
-			";
-			$orderby = 'post_count';
-		} elseif ( 'ID' == $qv['orderby'] || 'id' == $qv['orderby'] ) {
-			$orderby = 'ID';
-		} elseif ( 'meta_value' == $qv['orderby'] ) {
-			$orderby = "$wpdb->usermeta.meta_value";
-		} else {
-			$orderby = 'user_login';
+		if ( isset( $qv['orderby'] ) ) {
+			if ( in_array( $qv['orderby'], array('nicename', 'email', 'url', 'registered') ) ) {
+				$orderby = 'user_' . $qv['orderby'];
+			} elseif ( in_array( $qv['orderby'], array('user_nicename', 'user_email', 'user_url', 'user_registered') ) ) {
+				$orderby = $qv['orderby'];
+			} elseif ( 'name' == $qv['orderby'] || 'display_name' == $qv['orderby'] ) {
+				$orderby = 'display_name';
+			} elseif ( 'post_count' == $qv['orderby'] ) {
+				// todo: avoid the JOIN
+				$where = get_posts_by_author_sql('post');
+				$this->query_from .= " LEFT OUTER JOIN (
+					SELECT post_author, COUNT(*) as post_count
+					FROM $wpdb->posts
+					$where
+					GROUP BY post_author
+				) p ON ({$wpdb->users}.ID = p.post_author)
+				";
+				$orderby = 'post_count';
+			} elseif ( 'ID' == $qv['orderby'] || 'id' == $qv['orderby'] ) {
+				$orderby = 'ID';
+			} elseif ( 'meta_value' == $qv['orderby'] ) {
+				$orderby = "$wpdb->usermeta.meta_value";
+			} else {
+				$orderby = 'user_login';
+			}
 		}
 
-		$qv['order'] = strtoupper( $qv['order'] );
+		if ( empty( $orderby ) )
+			$orderby = 'user_login';
+
+		$qv['order'] = isset( $qv['order'] ) ? strtoupper( $qv['order'] ) : '';
 		if ( 'ASC' == $qv['order'] )
 			$order = 'ASC';
 		else
@@ -449,14 +454,17 @@
 		$this->query_orderby = "ORDER BY $orderby $order";
 
 		// limit
-		if ( $qv['number'] ) {
+		if ( isset( $qv['number'] ) && $qv['number'] ) {
 			if ( $qv['offset'] )
 				$this->query_limit = $wpdb->prepare("LIMIT %d, %d", $qv['offset'], $qv['number']);
 			else
 				$this->query_limit = $wpdb->prepare("LIMIT %d", $qv['number']);
 		}
 
-		$search = trim( $qv['search'] );
+		$search = '';
+		if ( isset( $qv['search'] ) )
+			$search = trim( $qv['search'] );
+
 		if ( $search ) {
 			$leading_wild = ( ltrim($search, '*') != $search );
 			$trailing_wild = ( rtrim($search, '*') != $search );
@@ -490,16 +498,20 @@
 			$this->query_where .= $this->get_search_sql( $search, $search_columns, $wild );
 		}
 
-		$blog_id = absint( $qv['blog_id'] );
+		$blog_id = 0;
+		if ( isset( $qv['blog_id'] ) )
+			$blog_id = absint( $qv['blog_id'] );
 
-		if ( 'authors' == $qv['who'] && $blog_id ) {
+		if ( isset( $qv['who'] ) && 'authors' == $qv['who'] && $blog_id ) {
 			$qv['meta_key'] = $wpdb->get_blog_prefix( $blog_id ) . 'user_level';
 			$qv['meta_value'] = 0;
 			$qv['meta_compare'] = '!=';
 			$qv['blog_id'] = $blog_id = 0; // Prevent extra meta query
 		}
 
-		$role = trim( $qv['role'] );
+		$role = '';
+		if ( isset( $qv['role'] ) )
+			$role = trim( $qv['role'] );
 
 		if ( $blog_id && ( $role || is_multisite() ) ) {
 			$cap_meta_query = array();
@@ -525,10 +537,10 @@
 				$this->query_fields = 'DISTINCT ' . $this->query_fields;
 		}
 
-		if ( !empty( $qv['include'] ) ) {
+		if ( ! empty( $qv['include'] ) ) {
 			$ids = implode( ',', wp_parse_id_list( $qv['include'] ) );
 			$this->query_where .= " AND $wpdb->users.ID IN ($ids)";
-		} elseif ( !empty($qv['exclude']) ) {
+		} elseif ( ! empty( $qv['exclude'] ) ) {
 			$ids = implode( ',', wp_parse_id_list( $qv['exclude'] ) );
 			$this->query_where .= " AND $wpdb->users.ID NOT IN ($ids)";
 		}
@@ -553,7 +565,7 @@
 			$this->results = $wpdb->get_col("SELECT $this->query_fields $this->query_from $this->query_where $this->query_orderby $this->query_limit");
 		}
 
-		if ( $qv['count_total'] )
+		if ( isset( $qv['count_total'] ) && $qv['count_total'] )
 			$this->total_users = $wpdb->get_var( apply_filters( 'found_users_query', 'SELECT FOUND_ROWS()' ) );
 
 		if ( !$this->results )
Index: tests/phpunit/tests/user/query.php
===================================================================
--- tests/phpunit/tests/user/query.php	(revision 25381)
+++ tests/phpunit/tests/user/query.php	(working copy)
@@ -20,7 +20,7 @@
 		$users = new WP_User_Query();
 
 		$this->assertEquals( '', $users->get( 'fields' ) );
-		$this->assertEquals( '', $users->query_vars['fields'] );
+		$this->assertEquals( '', @$users->query_vars['fields'] );
 
 		$users->set( 'fields', 'all' );