diff --git a/lib/filter.js b/lib/filter.js
index 4561f5c..00930be 100644
--- a/lib/filter.js
+++ b/lib/filter.js
@@ -106,7 +106,7 @@ module.exports = function filter(schema, options) {
         var filters = this._getFilterKeys("readFilter", filterRole);
         filters = filters ? filters.concat('_id') : filters; // Always send _id property
         if (options.compat) return filters;
-        else return filters.join(' ');
+	else return (filters) ? filters.join(' '): null;
     };
 
     schema.statics.getWriteFilterKeys = function(filterRole){