diff -r 4c65f8641d89 -r 125e080bbe15 Lib/test/test_wsgiref.py
--- a/Lib/test/test_wsgiref.py	Tue Apr 15 13:52:21 2014 +0100
+++ b/Lib/test/test_wsgiref.py	Tue Apr 15 14:33:29 2014 -0400
@@ -17,6 +17,16 @@
 
 from test import support
 
+
+# The server signature changes from version to version and we need it multiple
+# times later on. Precompute it to make the tests more readable.
+ver = sys.version.split()[0]
+py = python_implementation()
+pyver = py + "/" + ver
+EXPECTED_SERVER_SIGNATURE = (
+    "Server: WSGIServer/0.2 {}\r\n".format(pyver)).encode('iso-8859-1')
+
+
 class MockServer(WSGIServer):
     """Non-socket HTTP server"""
 
@@ -102,17 +112,16 @@
 class IntegrationTests(TestCase):
 
     def check_hello(self, out, has_length=True):
-        pyver = (python_implementation() + "/" +
-                sys.version.split()[0])
-        self.assertEqual(out,
-            ("HTTP/1.0 200 OK\r\n"
-            "Server: WSGIServer/0.2 " + pyver +"\r\n"
-            "Content-Type: text/plain\r\n"
-            "Date: Mon, 05 Jun 2006 18:49:54 GMT\r\n" +
-            (has_length and  "Content-Length: 13\r\n" or "") +
-            "\r\n"
-            "Hello, world!").encode("iso-8859-1")
-        )
+        expected = (
+            b"HTTP/1.0 200 OK\r\n" +
+            EXPECTED_SERVER_SIGNATURE +
+            b"Content-Type: text/plain\r\n" +
+            b"Date: Mon, 05 Jun 2006 18:49:54 GMT\r\n" +
+            (b"Content-Length: 13\r\n" if has_length else b"") +
+            b"\r\n"
+            b"Hello, world!")
+
+        self.assertEqual(out, expected)
 
     def test_plain_hello(self):
         out, err = run_amock()
@@ -159,17 +168,31 @@
             return [b"data"]
         out, err = run_amock(validator(app))
         self.assertTrue(err.endswith('"GET / HTTP/1.0" 200 4\n'))
-        ver = sys.version.split()[0].encode('ascii')
-        py  = python_implementation().encode('ascii')
-        pyver = py + b"/" + ver
         self.assertEqual(
-                b"HTTP/1.0 200 OK\r\n"
-                b"Server: WSGIServer/0.2 "+ pyver + b"\r\n"
-                b"Content-Type: text/plain; charset=utf-8\r\n"
-                b"Date: Wed, 24 Dec 2008 13:29:32 GMT\r\n"
-                b"\r\n"
-                b"data",
-                out)
+            b"HTTP/1.0 200 OK\r\n" +
+            EXPECTED_SERVER_SIGNATURE +
+            b"Content-Type: text/plain; charset=utf-8\r\n"
+            b"Date: Wed, 24 Dec 2008 13:29:32 GMT\r\n"
+            b"\r\n"
+            b"data",
+            out)
+
+    def test_304_correctly_does_not_set_content_length(self):
+        # bug 18099
+
+        def non_modified_app(environ, start_response):
+            start_response("304 Not Modified", [
+                ("Date", "Wed, 24 Dec 2008 13:29:32 GMT")])
+            return []
+
+        out, err = run_amock(validator(non_modified_app))
+
+        self.assertEqual(
+            b"HTTP/1.0 304 Not Modified\r\n" +
+            EXPECTED_SERVER_SIGNATURE +
+            b"Date: Wed, 24 Dec 2008 13:29:32 GMT\r\n" +
+            b"\r\n",
+            out)
 
 
 class UtilityTests(TestCase):
diff -r 4c65f8641d89 -r 125e080bbe15 Lib/wsgiref/handlers.py
--- a/Lib/wsgiref/handlers.py	Tue Apr 15 13:52:21 2014 +0100
+++ b/Lib/wsgiref/handlers.py	Tue Apr 15 14:33:29 2014 -0400
@@ -187,26 +187,30 @@
         """Return the URL scheme being used"""
         return guess_scheme(self.environ)
 
-
     def set_content_length(self):
         """Compute Content-Length or switch to chunked encoding if possible"""
         try:
             blocks = len(self.result)
-        except (TypeError,AttributeError,NotImplementedError):
+        except (TypeError, AttributeError, NotImplementedError):
             pass
         else:
-            if blocks==1:
+            if blocks in [0, 1]:
                 self.headers['Content-Length'] = str(self.bytes_sent)
                 return
         # XXX Try for chunked encoding if origin server and client is 1.1
 
-
     def cleanup_headers(self):
         """Make any necessary header changes or defaults
 
         Subclasses can extend this to add other defaults.
         """
-        if 'Content-Length' not in self.headers:
+        # RFC 2616, 4.3
+        allows_body = (not self.status.startswith('1') and
+                       not self.status.startswith('204') and
+                       not self.status.startswith('304'))
+        content_length_set = 'Content-Length' in self.headers
+
+        if allows_body and not content_length_set:
             self.set_content_length()
 
     def start_response(self, status, headers,exc_info=None):
@@ -300,16 +304,12 @@
         """
         return False   # No platform-specific transmission by default
 
-
     def finish_content(self):
         """Ensure headers and content have both been sent"""
         if not self.headers_sent:
-            # Only zero Content-Length if not set by the application (so
-            # that HEAD requests can be satisfied properly, see #3839)
-            self.headers.setdefault('Content-Length', "0")
             self.send_headers()
         else:
-            pass # XXX check if content-length was too short?
+            pass  # XXX check if content-length was too short?
 
     def close(self):
         """Close the iterable (if needed) and reset all instance vars