diff --git a/src/Document.js b/src/Document.js
index 9b170a5..c8cb540 100644
--- a/src/Document.js
+++ b/src/Document.js
@@ -65,6 +65,8 @@
   }
 
   count(xpath) {
+    if (typeof xpath !== 'string') throw new Error('xpath must be a string.');
+
     return this.get(`count(${xpath})`);
   }