diff --git a/index.js b/index.js
index e3ae013..ff8b4a8 100644
--- a/index.js
+++ b/index.js
@@ -14,6 +14,10 @@ module.exports = (function()
 
   return function(a, b)
   {
+    if (!a || !b) {
+      return;
+    }
+
     if (a === b) {
       return 0;
     }
diff --git a/test.js b/test.js
index e07a681..7b40d73 100644
--- a/test.js
+++ b/test.js
@@ -17,4 +17,5 @@ test(t =>
        t.is(levenshtein('levenshtein', 'frankenstein'), 6);
        t.is(levenshtein('distance', 'difference'), 5);
        t.is(levenshtein('因為我是中國人所以我會說中文', '因為我是英國人所以我會說英文'), 2);
+       t.is(levenshtein('Hello World', undefined), undefined);
      });