使用protobuf的架构i386的未定义符号

我正在尝试制作使用协议缓冲区的应用程序。我收到这个错误，为什么？使用protobuf的架构i386的未定义符号

Ld /Users/fmota/Library/Developer/Xcode/DerivedData/PBTest-gvudadeakgzklbekugyiqyfyprlt/Build/Products/Debug-iphonesimulator/PBTest.app/PBTest normal i386 
cd /Users/fmota/Documents/Developer/Protobuf/PBTest 
setenv MACOSX_DEPLOYMENT_TARGET 10.6 
setenv PATH "/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin:/Developer/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin" 
/Developer/Platforms/iPhoneSimulator.platform/Developer/usr/bin/clang -arch i386 -isysroot /Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator5.0.sdk -L/Users/fmota/Library/Developer/Xcode/DerivedData/PBTest-gvudadeakgzklbekugyiqyfyprlt/Build/Products/Debug-iphonesimulator -F/Users/fmota/Library/Developer/Xcode/DerivedData/PBTest-gvudadeakgzklbekugyiqyfyprlt/Build/Products/Debug-iphonesimulator -filelist /Users/fmota/Library/Developer/Xcode/DerivedData/PBTest-gvudadeakgzklbekugyiqyfyprlt/Build/Intermediates/PBTest.build/Debug-iphonesimulator/PBTest.build/Objects-normal/i386/PBTest.LinkFileList -mmacosx-version-min=10.6 -Xlinker -objc_abi_version -Xlinker 2 -Xlinker -no_implicit_dylibs -D__IPHONE_OS_VERSION_MIN_REQUIRED=50000 -framework SystemConfiguration -framework UIKit -framework Foundation -framework CoreGraphics -o /Users/fmota/Library/Developer/Xcode/DerivedData/PBTest-gvudadeakgzklbekugyiqyfyprlt/Build/Products/Debug-iphonesimulator/PBTest.app/PBTest 

Undefined symbols for architecture i386: 
    "_OBJC_CLASS_$_AreaRequest_Builder", referenced from: 
     objc-class-ref in ViewController.o 
ld: symbol(s) not found for architecture i386 
clang: error: linker command failed with exit code 1 (use -v to see invocation)

来源

2012-01-05 Filipe Mota