-[__NSDictionaryM urlEncodedKeyValueString]: unrecognized selector sent

[cczufish]

NSDictionary *pDic = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt:1], @"userId",
[NSNumber numberWithInt:1], @"skuId",
nil];
if (([method isEqualToString:@"GET"] ||
[method isEqualToString:@"DELETE"]) && (params && [params count] > 0)) {

finalURL = [NSURL URLWithString:[NSString stringWithFormat:@"%@?%@", aURLString,
[self.fieldsToBePosted urlEncodedKeyValueString]]];
}
-[__NSDictionaryM urlEncodedKeyValueString]: unrecognized selector sent to instance 0x170e4ec40

why