iOS - [Class classMethod] results in unrecognized

问题:

Due to code privacy, I have to omit part of info. Just excuse me.

In TokenRequestManager.m, there is an instance method serialQueue4TokenType:

- (dispatch_queue_t)serialQueue4TokenType:(TokenType)tokenType
{
    static NSMutableDictionary *serialQueueDict = nil;
    if (serialQueueDict == nil) {
        static dispatch_once_t onceToken;
        dispatch_once(&onceToken, ^{
            serialQueueDict = [[NSMutableDictionary alloc] initWithCapacity:TokenTypeCount];
        });
    }

    dispatch_queue_t queue;

    @synchronized (self) {
        NSString *key = [TokenManager tokenName4Type:tokenType];
        key = [key stringByAppendingString:NSStringFromClass([self class])];
        NSValue *value = serialQueueDict[key];

        if (value == nil) {
            queue = dispatch_queue_create(key.UTF8String, DISPATCH_QUEUE_SERIAL);
            value = [NSValue valueWithBytes:&queue objCType:@encode(dispatch_queue_t)];
            serialQueueDict[key] = value;
        } else {
            [value getValue:&queue];
        }
    }

    return queue;
}

Just below the @synchronized (self) { line, it calls the Class method:

+ (NSString *)tokenName4Type:(TokenType)tokenType
{
    NSString *string = nil;

    switch (tokenType) {
        case TokenTypeBiz:
            return @"TokenTypeBiz";
            break;

        case TokenTypeWeb:
        default:
            string = @"TokenTypeWeb";
            break;
    }

    return string;
}

The code just stands there, but sometimes (a few times) the app will crash at this line with unrecognized selector sent, no matter debug or release.

Any tips will be much appreciated. Thanks.