diff --git a/AFNetworking/AFHTTPRequestOperation.m b/AFNetworking/AFHTTPRequestOperation.m
index 98aa07c..80b61f8 100644
--- a/AFNetworking/AFHTTPRequestOperation.m
+++ b/AFNetworking/AFHTTPRequestOperation.m
@@ -165,7 +165,7 @@ static void AFSwizzleClassMethodWithClassAndSelectorUsingBlock(Class klass, SEL
 - (NSStringEncoding)responseStringEncoding {
     // When no explicit charset parameter is provided by the sender, media subtypes of the "text" type are defined to have a default charset value of "ISO-8859-1" when received via HTTP. Data in character sets other than "ISO-8859-1" or its subsets MUST be labeled with an appropriate charset value.
     // See http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.4.1
-    if (self.response && !self.response.textEncodingName && self.responseData) {
+    if (self.response && !self.response.textEncodingName && self.responseData && [self.response respondsToSelector:@selector(allHeaderFields)]) {
         NSString *type = nil;
         AFGetMediaTypeAndSubtypeWithString([[self.response allHeaderFields] valueForKey:@"Content-Type"], &type, nil);