diff --git a/AFNetworking/AFURLConnectionOperation.m b/AFNetworking/AFURLConnectionOperation.m
index 339f145..74d0c0b 100644
--- a/AFNetworking/AFURLConnectionOperation.m
+++ b/AFNetworking/AFURLConnectionOperation.m
@@ -325,12 +325,15 @@ static inline BOOL AFStateTransitionIsValid(AFOperationState fromState, AFOperat
 - (NSString *)responseString {
     [self.lock lock];
     if (!_responseString && self.response && self.responseData) {
-        NSStringEncoding textEncoding = NSUTF8StringEncoding;
+        NSStringEncoding stringEncoding = NSUTF8StringEncoding;
         if (self.response.textEncodingName) {
-            textEncoding = CFStringConvertEncodingToNSStringEncoding(CFStringConvertIANACharSetNameToEncoding((__bridge CFStringRef)self.response.textEncodingName));
+            CFStringEncoding IANAEncoding = CFStringConvertIANACharSetNameToEncoding((__bridge CFStringRef)self.response.textEncodingName);
+            if (IANAEncoding != kCFStringEncodingInvalidId) {
+                stringEncoding = CFStringConvertEncodingToNSStringEncoding(IANAEncoding);
+            }
         }
         
-        self.responseString = [[NSString alloc] initWithData:self.responseData encoding:textEncoding];
+        self.responseString = [[NSString alloc] initWithData:self.responseData encoding:stringEncoding];
     }
     [self.lock unlock];