做搜索栏的时候报mutating method sent to immutable object

跟着iOS5开发基础教程上面做示例，第8章有个搜索栏，我做出来之后就是没法搜索，后面断点跟踪了下。部分代码如下：
(void)handleSearchForItem:(NSString )searchItem{ NSMutableArray sectionsToRemove = [[NSMutableArray alloc] init]; [self resetSearch]; for (NSString
当程序走到[array removeObjectsInArray:toRemove];这行时就报错：WebKit discarded an uncaught exception in the webView:shouldInsertText:replacingDOMRange:givenAction: delegate: -[__NSCFArray removeObjectAtIndex:]: mutating method sent to