iOS/objective-c: Spooky error setting value of object - ios

Question

I have copied some code that works fine for saving an api to one entity in core data over to save a similar api to another entity. Although everything is largely the same, I cannot get rid of exception error.
Here is the code that throws the exception:
- (NSMutableArray *) convertFeedtoObject:(NSMutableArray*)feed {
NSMutableArray * _newitems = [[NSMutableArray alloc] init];
for (int i = 0; i < feed.count; i++) {
NSDictionary *feedElement = feed[i];
ItemOnServer *newItem = [[ItemOnServer alloc] init];
newItem.itemtitle = feedElement[#"itemtitle"]; //throws exception
newItem.item = feedElement[#"item"];//also throws an exception if above commented out
newItem.descript = #"some item"; //if above lines commented out, littoral throws exception too.
//same goes for any other values I try to set. The first one throws exception.
Here is the error:
2015-11-20 05:26:03.281 [20610:60b] -[ItemOnServer setItem:]: unrecognized selector sent to instance 0x175721b0
No matter what values I try to set, I get a similar error even though I know the values are there.
One item of the feed looks like this:
{
lastviewed = "2015-11-17 15:21:45";
itemtitle = "New";
id = 944;
item = "cotton shirt";
}
)
Thanks for any suggestions.
Edit:
Code from ItemOnServer.h
//.m file is largely empty
#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>
#class ItemsVC;
#class Vendors;
#interface ItemOnServer : NSObject
#property (nonatomic, retain) NSString * item;
#property (nonatomic, retain) NSNumber * id;//this is permanent id shared with server
#property (nonatomic, retain) NSNumber * localid; //this is temp id if first created locally
#property (nonatomic, retain) NSString * itemtitle;
#property (nonatomic, retain) NSDate * lastviewed;
//this is relationship with vendors
#property (nonatomic, retain) Vendors *vendor;
#end

Answer 1

Instead of using an NSDictionary declare feedElement as an NSMutableArray, so something like:
- (NSMutableArray *) convertFeedtoObject:(NSMutableArray*)feed {
NSMutableArray * _newitems = [[NSMutableArray alloc] init];
NSMutableArray *feedElement = [[NSMutableArray alloc] init];
for (int i = 0; i < feed.count; i++) {
feedElement = feed[i];
ItemOnServer *newItem = [[ItemOnServer alloc] init];
newItem.itemtitle = feedElement[#"itemtitle"];
...

Answer 2

It looks like your class ItemOnServer doesn't have implementations for the methods setItem:, setItemtitle and so on. I don't know how you managed to do this, but you should look at the interface and implementation of ItemOnServer.
Look at what the error message says:
-[ItemOnServer setItem:]: unrecognized selector
That means a setItem: message is sent to an object, that object is an ItemOnServer object, and it doesn't implement setItem: The first two of these three points are exactly what you expect to happen when you write newItem.item = ...
Do you have another class named ItemOnServer in your application, maybe linked through some library? You can easily check by renaming ItemOnServer with ItemOnServer2 and checking what happens.