diff --git a/xmlrpc.php b/xmlrpc.php
index 34a95a7d78..460721130d 100644
--- a/xmlrpc.php
+++ b/xmlrpc.php
@@ -1251,6 +1251,9 @@ class wp_xmlrpc_server extends IXR_Server {
 			}
 		}
 
+		if ( empty($context) )  // URL pattern not found
+			return new IXR_Error(17, 'The source URI does not contain a link to the target URI, and so cannot be used as a source.');
+
 		$pagelinkedfrom = preg_replace('#&([^amp\;])#is', '&$1', $pagelinkedfrom);
 
 		$context = '[...] ' . wp_specialchars( $excerpt ) . ' [...]';