Index: wp-includes/class-wp-admin-bar.php
===================================================================
--- wp-includes/class-wp-admin-bar.php	(revision 20826)
+++ wp-includes/class-wp-admin-bar.php	(working copy)
@@ -236,9 +236,11 @@
 			$group_class = ( $node->parent == 'root' ) ? 'ab-top-menu' : 'ab-submenu';
 
 			if ( $node->type == 'group' ) {
-				if ( empty( $node->meta['class'] ) )
-					$node->meta['class'] = '';
-				$node->meta['class'] .= ' ' . $group_class;
+				if ( ! empty( $node->meta['class'] ) ) {
+					$node->meta['class'] .= ' ' . $group_class;
+				} else {
+					$node->meta['class'] = $group_class;
+				}
 			}
 
 			// Items in items aren't allowed. Wrap nested items in 'default' groups.
@@ -393,9 +395,13 @@
 			$aria_attributes .= ' aria-haspopup="true"';
 		}
 
-		if ( ! empty( $node->meta['class'] ) )
-			$menuclass .= ' ' . $node->meta['class'];
-
+		if ( ! empty( $node->meta['class'] ) ) {
+			if ( ! empty( $menuclass ) ) {
+				$menuclass .= ' ' . $node->meta['class'];
+			} else {
+				$menuclass .= $node->meta['class'];
+			}
+		}
 		?>
 
 		<li id="<?php echo esc_attr( 'wp-admin-bar-' . $node->id ); ?>" class="<?php echo esc_attr( $menuclass ); ?>"><?php