diff --git a/fetch.js b/fetch.js
index 0beb1443..0e5d8e6a 100644
--- a/fetch.js
+++ b/fetch.js
@@ -7,7 +7,7 @@
 
   function normalizeName(name) {
     if (typeof name !== 'string') {
-      name = name.toString();
+      name = String(name)
     }
     if (/[^a-z0-9\-#$%&'*+.\^_`|~]/i.test(name)) {
       throw new TypeError('Invalid character in header field name')
@@ -17,7 +17,7 @@
 
   function normalizeValue(value) {
     if (typeof value !== 'string') {
-      value = value.toString();
+      value = String(value)
     }
     return value
   }
diff --git a/test/test.js b/test/test.js
index 768eb018..2b4f1008 100644
--- a/test/test.js
+++ b/test/test.js
@@ -128,7 +128,9 @@ suite('Headers', function() {
   test('converts field value to string on set and get', function() {
     var headers = new Headers()
     headers.set('Content-Type', 1)
+    headers.set('X-CSRF-Token', undefined);
     assert.equal(headers.get('Content-Type'), '1')
+    assert.equal(headers.get('X-CSRF-Token'), 'undefined')
   })
   test('throws TypeError on invalid character in field name', function() {
     assert.throws(function() { new Headers({'<Accept>': ['application/json']}) }, TypeError)