nsdictionry writing to File blank file getting created

Blank file getting created after saving Dictionary to a file.

The Cocoa API is very specific about what kind of values can legally be in a dictionary when it is written out to file. One particular limitation that has been known to cause problems and which is not thoroughly discussed in the official API documentation is that all of the keys in your dictionary must be of type NSString (and your values must be one of NSData, NSDate, NSNumber, NSString, NSArray, or NSDictionary), even though the dictionary itself supports keys and values of any object type.

from url: http://stackoverflow.com/questions/5361630/iphone-writing-nsmutabledictionary-to-file