diff --git a/AFNetworking/AFHTTPRequestOperation.m b/AFNetworking/AFHTTPRequestOperation.m
index 9d09e1c..1456eca 100644
--- a/AFNetworking/AFHTTPRequestOperation.m
+++ b/AFNetworking/AFHTTPRequestOperation.m
@@ -186,7 +186,15 @@ static NSString * AFStringFromIndexSet(NSIndexSet *indexSet) {
 		return NO;
 	}
     
-    return ![[self class] acceptableContentTypes] || [[[self class] acceptableContentTypes] containsObject:[self.response MIMEType]];
+    // According to RFC 2616:
+    // Any HTTP/1.1 message containing an entity-body SHOULD include a Content-Type header field defining the media type of that body. If and only if the media type is not given by a Content-Type field, the recipient MAY attempt to guess the media type via inspection of its content and/or the name extension(s) of the URI used to identify the resource. If the media type remains unknown, the recipient SHOULD treat it as type "application/octet-stream".
+    // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec7.html
+    NSString *contentType = [self.response MIMEType];
+    if (!contentType) {
+        contentType = @"application/octet-stream";
+    }
+    
+    return ![[self class] acceptableContentTypes] || [[[self class] acceptableContentTypes] containsObject:contentType];
 }
 
 - (void)setSuccessCallbackQueue:(dispatch_queue_t)successCallbackQueue {