diff --git a/index.js b/index.js
index d01f0d9..58cfd63 100644
--- a/index.js
+++ b/index.js
@@ -17,5 +17,5 @@ module.exports = contains;
  */
 
 function contains (node, other) {
-  return node === other || !!(node.compareDocumentPosition(other) & 16);
+  return other && (node === other || !!(node.compareDocumentPosition(other) & 16) );
 }