diff --git a/AFNetworking/AFHTTPRequestOperation.m b/AFNetworking/AFHTTPRequestOperation.m
index 871cb18..5e1dce9 100644
--- a/AFNetworking/AFHTTPRequestOperation.m
+++ b/AFNetworking/AFHTTPRequestOperation.m
@@ -135,6 +135,7 @@ static NSString * AFStringFromIndexSet(NSIndexSet *indexSet) {
     if (successCallbackQueue != _successCallbackQueue) {
         if (_successCallbackQueue) {
             dispatch_release(_successCallbackQueue);
+            _successCallbackQueue = NULL;
         }
      
         if (successCallbackQueue) {
@@ -148,6 +149,7 @@ static NSString * AFStringFromIndexSet(NSIndexSet *indexSet) {
     if (failureCallbackQueue != _failureCallbackQueue) {
         if (_failureCallbackQueue) {
             dispatch_release(_failureCallbackQueue);
+            _failureCallbackQueue = NULL;
         }
         
         if (failureCallbackQueue) {