diff --git a/index.js b/index.js
index 7ac922bc..a45e0eda 100644
--- a/index.js
+++ b/index.js
@@ -155,10 +155,10 @@ micromatch.not = (list, patterns, options = {}) => {
     items.push(state.output);
   };
 
-  let matches = micromatch(list, patterns, { ...options, onResult });
+  let matches = new Set(micromatch(list, patterns, { ...options, onResult }));
 
   for (let item of items) {
-    if (!matches.includes(item)) {
+    if (!matches.has(item)) {
       result.add(item);
     }
   }