diff --git a/trac/web/api.py b/trac/web/api.py
index 383db0a067..7f8b59bdcb 100644
--- a/trac/web/api.py
+++ b/trac/web/api.py
@@ -982,6 +982,8 @@ def write(self, data):
         there are multiple calls to `write`, to the cumulative length
         of the *data* arguments.
         """
+        if isinstance(data, str):
+            raise ValueError("Can't send str content")
         if not self._write:
             self.end_headers()
         try:
@@ -1028,7 +1030,7 @@ def _send(self, content, content_type='text/html', status=200,
         self.send_header('Cache-Control', 'must-revalidate')
         self.send_header('Expires', 'Fri, 01 Jan 1999 00:00:00 GMT')
         self.send_header('Content-Type', content_type + ';charset=utf-8')
-        if isinstance(content, str):
+        if isinstance(content, bytes):
             self.send_header('Content-Length', len(content))
         self.end_headers(exc_info)
 
diff --git a/trac/web/tests/api.py b/trac/web/tests/api.py
index 01010e120d..815fb73c6e 100644
--- a/trac/web/tests/api.py
+++ b/trac/web/tests/api.py
@@ -440,8 +440,23 @@ def test_write_unicode(self):
         # anyway we're not supposed to send unicode, so we get a ValueError
         with self.assertRaises(ValueError):
             req.write('Föö')
+        with self.assertRaises(ValueError):
+            req.write('')
         with self.assertRaises(ValueError):
             req.write((b'F', 'öo'))
+        with self.assertRaises(ValueError):
+            req.write(('Föo'.encode('utf-8'), ''))
+
+    def test_send_bytes(self):
+        req = _make_req(_make_environ(method='GET'))
+        with self.assertRaises(RequestDone):
+            req.send(b'\xef\xbb\xbf')
+        self.assertEqual('3', req.headers_sent.get('Content-Length'))
+
+    def test_send_unicode(self):
+        req = _make_req(_make_environ(method='GET'))
+        with self.assertRaises(ValueError):
+            req.send(u'\ufeff')
 
     def test_send_iterable(self):
         def iterable():
diff --git a/trac/web/tests/main.py b/trac/web/tests/main.py
index 1dfdd81641..d1cfbb3386 100644
--- a/trac/web/tests/main.py
+++ b/trac/web/tests/main.py
@@ -90,7 +90,7 @@ def match_request(self, req):
             def process_request(self, req):
                 self.calls += 1
                 req.authname
-                req.send('')
+                req.send(b'')
 
         cls.authenticators['success1'] = SuccessfulAuthenticator1
         cls.authenticators['success2'] = SuccessfulAuthenticator2