diff --git a/lib/adapters/memory.js b/lib/adapters/memory.js
index 314b1555..fbd748cc 100644
--- a/lib/adapters/memory.js
+++ b/lib/adapters/memory.js
@@ -130,6 +130,8 @@ function applyFilter(filter) {
         if (typeof value === 'string' && example && example.constructor.name === 'RegExp') {
             return value.match(example);
         }
+        if (typeof example === 'undefined') return undefined;
+        if (typeof value === 'undefined') return undefined;
         // not strict equality
         return (example !== null ? example.toString() : example) == (value !== null ? value.toString() : value);
     }