diff --git a/lib/unexpectedMap.js b/lib/unexpectedMap.js
index 3cea8fa..32e4ffd 100644
--- a/lib/unexpectedMap.js
+++ b/lib/unexpectedMap.js
@@ -507,11 +507,7 @@ function createUnexpectedMap(expect) {
   );
 
   expect.addAssertion('<Map> [not] to have size <number>', (expect, subject, expectedSize) => {
-    if (expect.flags.not) {
-      expect(subject.size === expectedSize, 'to be false');
-    } else {
-      expect(subject.size === expectedSize, 'to be true');  
-    }
+    expect(subject.size, '[not] to equal', expectedSize);
   });
 }