keysSortedByValueUsingSelector crashes but sortedArrayUsingSelector runs fine - iphone

Question

I found a workaround myself, but still trying to understand the problem.
I created a Autocomplete text field with the use of uitableview which is hidden until textfield is edited. The UI part works fine. It's the searching for the results part that's the problem. I declared a local NSMutableDictionary to store my results because I wanted the results to be sorted by the key's values.
if I call keysSortedByValueUsingSelector on the dictionary directly, it crashes. However if I get the keys by [dict allKeys] first, then call sortedArrayUsingSelector, it works fine:
// This commented out line will crash
// NSArray *sortedKeysArray = [dict keysSortedByValueUsingSelector:#selector(compare:)];
// The next two lines runs fine.
NSArray *keyArray = [dict allKeys];
NSArray *sortedKeysArray = [keyArray sortedArrayUsingSelector:#selector(compare:)];
Here is the complete source code for the search method:
- (void)searchAutocompleteEntriesWithSubstring:(NSString *)substring
{
// Put anything that starts with this substring into the autocompleteUrls array
// The items in this array is what will show up in the table view
[autocomplete_symbol_array removeAllObjects];
rRSIAppDelegate *appDelegate = (rRSIAppDelegate *)([[UIApplication sharedApplication] delegate]);
NSString *input_str = [substring uppercaseString];
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
int i = 0;
for(SymbolInfo *symbol_info in appDelegate.m_symbol_info_array)
{
i++;
NSString *info_str = [[[symbol_info.m_symbol uppercaseString] stringByAppendingString:#"|"] stringByAppendingString:[symbol_info.m_company_name uppercaseString]];
NSUInteger pos = [info_str rangeOfString:input_str].location;
if (pos != NSNotFound)
{
int tmp = pos * 10000 + i;
NSNumber *map_key = [[NSNumber alloc] initWithInt:tmp];
[dict setObject:symbol_info forKey:map_key];
}
}
// This commented out line will crash
// NSArray *sortedKeysArray = [dict keysSortedByValueUsingSelector:#selector(compare:)];
// The next two lines runs fine.
NSArray *keyArray = [dict allKeys];
NSArray *sortedKeysArray = [keyArray sortedArrayUsingSelector:#selector(compare:)];
for (NSNumber *key in sortedKeysArray)
{
SymbolInfo *symbol_info = [dict objectForKey:key];
[autocomplete_symbol_array addObject:symbol_info];
}
// NSLog(#"everything added: %d", [autocomplete_symbol_array count]);
[autocompleteTableView reloadData];
}

Answer 1

The NSMutableDictionary's method is:
- (void)setObject:(id)anObject forKey:(id < NSCopying >)aKey;
This means that the key should implement the NSCopying protocol.