NSPropertyListSerialization returns nil when trying to convert NSDictionary to NSData

I'm trying to convert an NSDictionary to NSData to store in core data, but I am getting a nil value returned to me. error states 'Unknown format option' after the call.

NSString *error = nil;

NSData *d = [NSPropertyListSerialization dataFromPropertyList:data format:NSPropertyListImmutable errorDescription:&error];

The data I'm trying to convert is an NSDictionary with NSStrings as keys and values. Here is a dump of the data dictionary:

Printing description of data:

<CFBasicHash 0xc96fd60 [0x18ecb38]>{type = immutable dict, count = 4,

entries =>

0 : <CFString 0x17c9fc [0x18ecb38]>{contents = "title"} = <CFString 0xecc8040 [0x18ecb38]>{contents = "test"}

1 : <CFString 0x17ca2c [0x18ecb38]>{contents = "author"} = <CFString 0xc9643f0 [0x18ecb38]>{contents = "test"}

2 : <CFString 0x17ca0c [0x18ecb38]>{contents = "goal"} = <CFString 0xc96f730 [0x18ecb38]>{contents = "test"}

4 : <CFString 0x17ca1c [0x18ecb38]>{contents = "description"} = <CFString 0xc9643f0 [0x18ecb38]>{contents = "test"}

}

Printing description of error:

Unknown format option

Any ideas on where I went wrong or alternate ways to convert an NSDictionary to NSData?