Linker command error: 'ld: symbol(s) not found for architecture i386'

I have an error at compile time. Seems to be weird. it worked as detail view application, but using single view doesn't seem to work. this is the error details:

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