diff --git a/AFNetworking/AFURLRequestSerialization.m b/AFNetworking/AFURLRequestSerialization.m
index 68c2c2d7be..15bb02d2d2 100644
--- a/AFNetworking/AFURLRequestSerialization.m
+++ b/AFNetworking/AFURLRequestSerialization.m
@@ -312,7 +312,7 @@ - (NSDictionary *)HTTPRequestHeaders {
 - (void)setValue:(NSString *)value
 forHTTPHeaderField:(NSString *)field
 {
-    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
+    dispatch_barrier_sync(self.requestHeaderModificationQueue, ^{
         [self.mutableHTTPRequestHeaders setValue:value forKey:field];
     });
 }