diff --git a/trac/web/api.py b/trac/web/api.py
index 95a060ce03..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:
diff --git a/trac/web/tests/api.py b/trac/web/tests/api.py
index 589dd8650a..815fb73c6e 100644
--- a/trac/web/tests/api.py
+++ b/trac/web/tests/api.py
@@ -440,8 +440,12 @@ 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'))
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