diff --git a/docs/topics/request-response.rst b/docs/topics/request-response.rst
index 3472b1937ad88a295764903b2e11287e117240cc..9f4dcdc46556d68743041946dc4b9c38f45bba97 100644
--- a/docs/topics/request-response.rst
+++ b/docs/topics/request-response.rst
@@ -466,12 +466,14 @@ TextResponse objects
 
     .. attribute:: TextResponse.encoding
 
-       A string with the encoding of this response. The encoding is resolved in the
-       following order: 
+       A string with the encoding of this response. The encoding is resolved by
+       trying the following mechanisms, in order:
 
        1. the encoding passed in the constructor `encoding` argument
 
-       2. the encoding declared in the Content-Type HTTP header
+       2. the encoding declared in the Content-Type HTTP header. If this
+          encoding is not valid (ie. unknown), it is ignored and the next
+          resolution mechanism is tried.
 
        3. the encoding declared in the response body. The TextResponse class
           doesn't provide any special functionality for this. However, the
diff --git a/scrapy/http/response/text.py b/scrapy/http/response/text.py
index 1c13e729cfdb2bbab8316600108db287c42de3b3..d42a891972716011538cee0e5b0d676514802196 100644
--- a/scrapy/http/response/text.py
+++ b/scrapy/http/response/text.py
@@ -5,6 +5,7 @@ discovering (through HTTP headers) to base Response class.
 See documentation in docs/topics/request-response.rst
 """
 
+import codecs
 import re
 
 from scrapy.xlib.BeautifulSoup import UnicodeDammit
@@ -64,7 +65,12 @@ class TextResponse(Response):
         if content_type:
             encoding = self._ENCODING_RE.search(content_type)
             if encoding:
-                return encoding.group(1)
+                enc = encoding.group(1)
+                try:
+                    codecs.lookup(enc) # check if the encoding is valid
+                    return enc
+                except LookupError:
+                    pass
 
     @memoizemethod_noargs
     def body_as_unicode(self):
diff --git a/scrapy/tests/test_http_response.py b/scrapy/tests/test_http_response.py
index 3b0a144d2532c75d00d899d038c232d4deca8d52..5851572ac017410dd13c85b74e276e332b613b4f 100644
--- a/scrapy/tests/test_http_response.py
+++ b/scrapy/tests/test_http_response.py
@@ -175,6 +175,8 @@ class TextResponseTest(BaseResponseTest):
         r2 = self.response_class("http://www.example.com", encoding='utf-8', body=u"\xa3")
         r3 = self.response_class("http://www.example.com", headers={"Content-type": ["text/html; charset=iso-8859-1"]}, body="\xa3")
         r4 = self.response_class("http://www.example.com", body="\xa2\xa3")
+        r5 = self.response_class("http://www.example.com",
+        headers={"Content-type": ["text/html; charset=None"]}, body="\xc2\xa3")
 
         self.assertEqual(r1.headers_encoding(), "utf-8")
         self.assertEqual(r2.headers_encoding(), None)
@@ -182,6 +184,8 @@ class TextResponseTest(BaseResponseTest):
         self.assertEqual(r3.headers_encoding(), "iso-8859-1")
         self.assertEqual(r3.encoding, 'iso-8859-1')
         self.assertEqual(r4.headers_encoding(), None)
+        self.assertEqual(r5.headers_encoding(), None)
+        self.assertEqual(r5.encoding, "utf-8")
         assert r4.body_encoding() is not None and r4.body_encoding() != 'ascii'
         self._assert_response_values(r1, 'utf-8', u"\xa3")
         self._assert_response_values(r2, 'utf-8', u"\xa3")