NsMutable Array problem

when i remove object from nsmutable array it shows exception but some time it works

[Array removeObjectAtIndex:row];

i have use this code

Terminating app due to uncaught exception 'NSInternalInconsistencyException',开发者_开发技巧 reason: '-[__NSCFArray removeObjectAtIndex:]: mutating method sent to immutable object'

Array is type of NSarray ... it is object of NSArray it should be object of NSMutableArray and not NSArray