diff --git a/AFNetworking/AFHTTPClient.m b/AFNetworking/AFHTTPClient.m
index a48ae00..22bf41e 100644
--- a/AFNetworking/AFHTTPClient.m
+++ b/AFNetworking/AFHTTPClient.m
@@ -52,7 +52,7 @@ static NSString * const kAFMultipartFormBoundary = @"Boundary+0xAbCdEfGbOuNdArY"
 static NSUInteger const kAFHTTPClientDefaultMaxConcurrentOperationCount = 4;
 
 static NSString * AFBase64EncodedStringFromString(NSString *string) {
-    NSData *data = [NSData dataWithBytes:[string UTF8String] length:[string length]];
+    NSData *data = [NSData dataWithBytes:[string UTF8String] length:[string lengthOfBytesUsingEncoding:NSUTF8StringEncoding]];
     NSUInteger length = [data length];
     NSMutableData *mutableData = [NSMutableData dataWithLength:((length + 2) / 3) * 4];