Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 19940)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -3293,7 +3293,9 @@
 			}
 		}
 
-		$post_excerpt = isset( $content_struct['mt_excerpt'] ) ? $content_struct['mt_excerpt'] : null;
+		if ( isset( $content_struct['mt_excerpt'] ) )
+			$post_excerpt = $content_struct['mt_excerpt'];
+			
 		$post_more = isset( $content_struct['mt_text_more'] ) ? $content_struct['mt_text_more'] : null;
 
 		$post_status = $publish ? 'publish' : 'draft';