NSMutableArray changes to NSCFString

Question

When I try to add some string values to a NSMutableArray it shows following exception.

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSCFString addObject:]: unrecognized selector sent to instance

here is my code:

NSMutableArray *pTitle = [[NSMutableArray alloc]init];

code for adding object:

[pTitle addObject:txtTitle.text];

[dataBase setObject:pTitle forKey:@"Title"];

All I need to revoke the NSMutableArray. How can I resolve this... Thanks in advance for your Valuable helps...