AWS iOS Cognito Sync Error - Unrecognized Selector Sent To Class

I have been working on this issue for some time. In the process of debugging and authentication starting to work, Cognito Sync began to fail.

My code:

AWSCognito *syncClient = [AWSCognito defaultCognito];

fails with the error:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '+[AWSEndpoint endpointWithRegion:service:]: unrecognized selector sent to class

The headers used:

#import <AWSiOSSDKv2/AWSCore.h>
#import <AWSCognitoSync/Cognito.h>

I am using SDK 2.0.17.

This code worked while users were unauthenticated. Once authentication started working, I get this error. I also upgraded from SDK 2.0.8 to 2.0.17 during the authentication debug process. Any ideas out there?