In App Purchase Not Working in iOS

Hi, I am using this method for making In-app-purchase in IOS. But I am getting the following error.

Terminating app due to uncaught exception ‘NSInvalidArgumentException’, reason: ‘Invalid product identifier: (null)’
*** First throw call stack:
(0x2501ef8f 0x336cfc8b 0x2501eed5 0x2861e703 0x75d8f 0x91a63 0x9135f 0x927cb 0x25d635cf 0x24fe4d8f 0x24fe419f 0x24fe2805 0x24f2ef31 0x24f2ed43 0x2ca09201 0x28700879 0x58c4b 0x33c81aaf)
libc++abi.dylib: terminating with uncaught exception of type NSException

Please help me. I have implemented in many ways but couldn’t get the answer. Thanks in Advance

Can any-one please suggest a fix for this. I have been struggling with this issue.