diff --git a/fetch.js b/fetch.js
index c1cab8eedab19eaf750672b49c5c39d5ca58df45..283a3d7147cf8b45b4856c8e350f0921eef84dea 100644
--- a/fetch.js
+++ b/fetch.js
@@ -5,6 +5,23 @@
     return
   }
 
+  function normalizeName(name) {
+    if (typeof name !== 'string') {
+      name = name.toString();
+    }
+    if (/[^a-z0-9\-#$%&'*+.\^_`|~]/i.test(name)) {
+      throw new TypeError('Invalid character in header field name')
+    }
+    return name.toLowerCase()
+  }
+
+  function normalizeValue(value) {
+    if (typeof value !== 'string') {
+      value = value.toString();
+    }
+    return value
+  }
+
   function Headers(headers) {
     this.map = {}
 
@@ -24,7 +41,8 @@
   }
 
   Headers.prototype.append = function(name, value) {
-    name = name.toLowerCase()
+    name = normalizeName(name)
+    value = normalizeValue(value)
     var list = this.map[name]
     if (!list) {
       list = []
@@ -34,24 +52,24 @@
   }
 
   Headers.prototype['delete'] = function(name) {
-    delete this.map[name.toLowerCase()]
+    delete this.map[normalizeName(name)]
   }
 
   Headers.prototype.get = function(name) {
-    var values = this.map[name.toLowerCase()]
+    var values = this.map[normalizeName(name)]
     return values ? values[0] : null
   }
 
   Headers.prototype.getAll = function(name) {
-    return this.map[name.toLowerCase()] || []
+    return this.map[normalizeName(name)] || []
   }
 
   Headers.prototype.has = function(name) {
-    return this.map.hasOwnProperty(name.toLowerCase())
+    return this.map.hasOwnProperty(normalizeName(name))
   }
 
   Headers.prototype.set = function(name, value) {
-    this.map[name.toLowerCase()] = [value]
+    this.map[normalizeName(name)] = [normalizeValue(value)]
   }
 
   // Instead of iterable for now.
@@ -281,6 +299,7 @@
       }
 
       xhr.open(self.method, self.url, true)
+
       if ('responseType' in xhr && support.blob) {
         xhr.responseType = 'blob'
       }
diff --git a/test/test.js b/test/test.js
index 03b6036c39b2604e0c9cd0c6c298819d260664d5..0e62add96dba004fa6d3437a53f20dc47c77a7ce 100644
--- a/test/test.js
+++ b/test/test.js
@@ -44,13 +44,78 @@ test('rejects promise for network error', function() {
 
 // https://fetch.spec.whatwg.org/#headers-class
 suite('Headers', function() {
-  test('headers are case insensitve', function() {
+  test('headers are case insensitive', function() {
     var headers = new Headers({'Accept': 'application/json'})
     assert.equal(headers.get('ACCEPT'), 'application/json')
     assert.equal(headers.get('Accept'), 'application/json')
     assert.equal(headers.get('accept'), 'application/json')
   })
-})
+  test('appends to existing', function() {
+    var headers = new Headers({'Accept': 'application/json'})
+    assert.isFalse(headers.has('Content-Type'))
+    headers.append('Content-Type', 'application/json')
+    assert.isTrue(headers.has('Content-Type'))
+    assert.equal(headers.get('Content-Type'), 'application/json')
+  })
+  test('appends values to existing header name', function() {
+    var headers = new Headers({'Accept': 'application/json'})
+    headers.append('Accept', 'text/plain')
+    assert.equal(headers.getAll('Accept').length, 2)
+    assert.equal(headers.getAll('Accept')[0], 'application/json')
+    assert.equal(headers.getAll('Accept')[1], 'text/plain')
+  })
+  test('sets header name and value', function() {
+    var headers = new Headers()
+    headers.set('Content-Type', 'application/json')
+    assert.equal(headers.get('Content-Type'), 'application/json')
+  })
+  test('returns null on no header found', function() {
+    var headers = new Headers()
+    assert.isNull(headers.get('Content-Type'))
+  })
+  test('has headers that are set', function() {
+    var headers = new Headers()
+    headers.set('Content-Type', 'application/json')
+    assert.isTrue(headers.has('Content-Type'))
+  })
+  test('deletes headers', function() {
+    var headers = new Headers()
+    headers.set('Content-Type', 'application/json')
+    assert.isTrue(headers.has('Content-Type'))
+    headers.delete('Content-Type')
+    assert.isFalse(headers.has('Content-Type'))
+    assert.isNull(headers.get('Content-Type'))
+  })
+  test('returns list on getAll when header found', function() {
+    var headers = new Headers({'Content-Type': 'application/json'})
+    assert.isArray(headers.getAll('Content-Type'))
+    assert.equal(headers.getAll('Content-Type').length, 1)
+    assert.equal(headers.getAll('Content-Type')[0], 'application/json')
+  })
+  test('returns empty list on getAll when no header found', function() {
+    var headers = new Headers()
+    assert.isArray(headers.getAll('Content-Type'))
+    assert.equal(headers.getAll('Content-Type').length, 0)
+  })
+  test('converts field name to string on set and get', function() {
+    var headers = new Headers()
+    headers.set(1, 'application/json')
+    assert.equal(headers.get(1), 'application/json')
+  })
+  test('converts field value to string on set and get', function() {
+    var headers = new Headers()
+    headers.set('Content-Type', 1)
+    assert.equal(headers.get('Content-Type'), '1')
+  })
+  test('throws TypeError on invalid character in field name', function() {
+    assert.throws(function() { new Headers({'<Accept>': ['application/json']}) }, TypeError)
+    assert.throws(function() { new Headers({'Accept:': ['application/json']}) }, TypeError)
+    assert.throws(function() { 
+      var headers = new Headers();
+      headers.set({field: 'value'}, 'application/json');
+    }, TypeError)
+  })
+ })
 
 // https://fetch.spec.whatwg.org/#request-class
 suite('Request', function() {