diff --git a/lib/prune.js b/lib/prune.js
index 15b1cc8..899ff2f 100644
--- a/lib/prune.js
+++ b/lib/prune.js
@@ -9,7 +9,7 @@ function prune(doc, model, type) {
 
 
     for (var prop in doc) {
-        if (!prop || doc[prop] === undefined)
+        if (!type || !prop || doc[prop] === undefined)
             continue;
 
         var propDef = type[prop];
@@ -53,4 +53,4 @@ module.exports = function(model, collection, docs) {
     for (var i in docs) {
         prune(docs, model, entityType);
     }
-};
\ No newline at end of file
+};