Type mismatch exception. Could not convert parameter 'limit' to the required type. (400, E0000037)

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Hello,
Im attemtping to list the users in my org using the .NET Core SDK as follows:

 Okta.Sdk.ICollectionClient<IUser> oktaUsers = _oktaClient.Users.ListUsers(model.Email,null,10);
                                if (oktaUsers.Count<IUser>().Id > 0)
                                {
                                    _logger.LogWarning("Okta user was already setup. getting the first user ");
                                    oktaUser = await oktaUsers.First<IUser>();
                                    _logger.LogWarning("Okta user was already setup. the first user id found was: " + oktaUser.Id);
                                }

The following error occurs when the program hits the following line:
oktaUser = await oktaUsers.First<IUser>();

The error returned is:
Type mismatch exception. Could not convert parameter ‘limit’ to the required type. (400, E0000037)

Can anyone tell me what Im doing wrong?

Thank you kindly.
Adam