unrecognized selector sent to instance exception in iOS7

I have a requirement to check whether a String contains a particular string or not.For that purpose i am using a function of NSString class called ccontainsstring. This function works fine in ios8 without throwing any kind of exception but in case of ios7 it is throwing an exception called

 unrecognized selector sent to instance 

I have used try & catch block to prevent the exception but may i know why this is happening?

Thanks in advance