diff --git a/scrapy/trunk/scrapy/http/response.py b/scrapy/trunk/scrapy/http/response.py
index 0ec3a2f4b..594f684d7 100644
--- a/scrapy/trunk/scrapy/http/response.py
+++ b/scrapy/trunk/scrapy/http/response.py
@@ -29,10 +29,11 @@ class Response(object) :
         self.original_url = Url(original_url) if original_url else url # different if redirected or escaped
         self.headers = Headers(headers or {})
         self.status = status
-        if isinstance(body, str):
-            self.body = ResponseBody(body, self.headers_encoding())
-        else:
+        # FIXME this should be replaced with assert(isinstance(body, basestring)) since ResponseBody is not meant to be used directly
+        if isinstance(body, ResponseBody):
             self.body = body
+        else:
+            self.body = ResponseBody(body, self.headers_encoding())
         self.cached = False
         self.request = None # request which originated this response