app crash when try get user photo by Facebook iOS Graph API with app-scoped user id

I use Facebook for iOS. I get user's data without problem by Graph Api. From user's data I get app-scoped user id. Then I try get user's picture but application crashed. I tried use [FBRequestConnection startWithGraph:@"/me/picture" ...] and [FBRequestConnection startWithGraph:@"/(app id from user's data)/picture" ...] but still crashed. Why?

   2014-06-02 16:16:51.669 testapp[448:60b] -[NSNull dataUsingEncoding:]: unrecognized selector sent to instance 0x19f8068

   2014-06-02 16:16:51.679 testapp[448:60b] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[NSNull dataUsingEncoding:]: unrecognized selector sent to instance 0x19f8068'

   *** First throw call stack:

(
    0   CoreFoundation                      0x018ac1e4 __exceptionPreprocess + 180
    1   libobjc.A.dylib                     0x0162b8e5 objc_exception_throw + 44
    2   CoreFoundation                      0x01949243 -[NSObject(NSObject) doesNotRecognizeSelector:] + 275
    3   CoreFoundation                      0x0189c50b ___forwarding___ + 1019
    4   CoreFoundation                      0x0189c0ee _CF_forwarding_prep_0 + 14
    5   testapp                             0x00022721 +[FBUtility simpleJSONDecode:error:] + 42
    6   testapp                             0x0000a75e -[FBRequestConnection parseJSONOrOtherwise:error:] + 69
    7   testapp                             0x0000a5a0 -[FBRequestConnection parseJSONResponse:error:statusCode:] + 1256
    8   testapp                             0x00009e04 -[FBRequestConnection completeWithResponse:data:orError:] + 666
    9   testapp                             0x00007564 __68-[FBRequestConnection startWithCacheIdentity:skipRoundtripIfCached:]_block_invoke + 201
    10  testapp                             0x00045fbc -[FBURLConnection invokeHandler:error:response:responseData:] + 49
    11  testapp                             0x00045ec7 -[FBURLConnection logAndInvokeHandler:response:responseData:] + 414
    12  testapp                             0x00046442 -[FBURLConnection connectionDidFinishLoading:] + 248
    13  Foundation                          0x013f4e49 ___NSURLConnectionDidFinishLoading_block_invoke + 40
    14  Foundation                          0x0138b7e1 __65-[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:]_block_invoke + 62
    15  Foundation                          0x01213f5e -[NSURLConnectionInternalConnection invokeForDelegate:] + 119
    16  Foundation                          0x01213ec6 -[NSURLConnectionInternal _withConnectionAndDelegate:onlyActive:] + 208
    17  Foundation                          0x01213dd8 -[NSURLConnectionInternal _withActiveConnectionAndDelegate:] + 76
    18  Foundation                          0x01214188 _NSURLConnectionDidFinishLoading + 43
    19  CFNetwork                           0x064aa69f ___ZN27URLConnectionClient_Classic26_delegate_didFinishLoadingEU13block_pointerFvvE_block_invoke + 111
    20  CFNetwork                           0x064a83de ___ZN27URLConnectionClient_Classic18_withDelegateAsyncEPKcU13block_pointerFvP16_CFURLConnectionPK33CFURLConnectionClientCurrent_VMaxE_block_invoke_2 + 104
    21  CoreFoundation                      0x0184dc69 CFArrayApplyFunction + 57
    22  CFNetwork                           0x06411441 _ZN19RunloopBlockContext7performEv + 155
    23  CFNetwork                           0x064f33f4 _ZThn16_N19RunloopBlockContext24multiplexerClientPerformEv + 20
    24  CFNetwork                           0x06411257 _ZN17MultiplexerSource7performEv + 299
    25  CFNetwork                           0x0641106c _ZN17MultiplexerSource8_performEPv + 76
    26  CoreFoundation                      0x0183577f __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 15
    27  CoreFoundation                      0x0183510b __CFRunLoopDoSources0 + 235
    28  CoreFoundation                      0x018521ae __CFRunLoopRun + 910
    29  CoreFoundation                      0x018519d3 CFRunLoopRunSpecific + 467
    30  CoreFoundation                      0x018517eb CFRunLoopRunInMode + 123
    31  GraphicsServices                    0x038a05ee GSEventRunModal + 192
    32  GraphicsServices                    0x038a042b GSEventRun + 104
    33  UIKit                               0x002ebf9b UIApplicationMain + 1225
    34  testapp                             0x00003b2d main + 141
    35  libdyld.dylib                       0x01ef3701 start + 1
    36  ???                                 0x00000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)

Thanks!

I guess this is because you don't handle the http redirect Facebook is sending if you query /me/picture. If you just want the URL of the picture, use /me/picture?redirect=false as stated in the docs: https://developers.facebook.com/docs/graph-api/reference/v2.0/user/picture/#read

Otherwise add some code to handle http redirects.