swift keychain add, delete, get working, update not working - swift

Question

I'm troubleshooting a problem with Keychain updates. I have SecItemAdd, SecItemCopyMatching, and SecItemDelete working. But I'm stumped on SecItemUpdate. I always get a -50 fail.
I've put a public MacOS keychainTest project on Github at:
https://github.com/darrellroot/keychainTest/blob/master/README.md
FYI this almost identical question/answer should help me, but it's not:
Swift 4 Keychain SecItemUpdate Error status code -50
Here's relevant code snippets:
let keychainTest = "Keychain Test"
let serverHostname = "mail.nowhere.com"
let keychainService = "KeychainService"
AddQuery works:
let addQuery: [String: Any] = [kSecClass as String: kSecClassGenericPassword,
kSecAttrService as String: keychainService,
kSecAttrAccount as String: serverHostname,
kSecValueData as String: inputTextOutlet.stringValue]
let status = SecItemAdd(addQuery as CFDictionary, nil)
GetQuery works:
let getQuery: [String: Any] = [
kSecClass as String: kSecClassGenericPassword,
kSecAttrService as String: keychainService,
kSecAttrAccount as String: serverHostname,
kSecMatchLimit as String: kSecMatchLimitOne,
kSecReturnData as String: true]
var rawResult: AnyObject?
let status = SecItemCopyMatching(getQuery as CFDictionary, &rawResult)
statusLabel.stringValue = status.description
guard let retrievedData = rawResult as? Data else { return }
guard let pass = String(data: retrievedData, encoding: String.Encoding.utf8) else { return }
DeleteQuery works:
let deleteQuery: [String: Any] = [kSecClass as String: kSecClassGenericPassword,
kSecAttrService as String: keychainService,
kSecAttrAccount as String: serverHostname]
let status = SecItemDelete(deleteQuery as CFDictionary)
statusLabel.stringValue = status.description
But for this updateQuery I always get a -50 OSStatus message:
let updateQuery: [String: Any] = [kSecClass as String: kSecClassGenericPassword,
kSecAttrService as String: keychainService,
kSecAttrAccount as String: serverHostname]
let newAttributes: [String: Any] = [kSecValueData as String: inputTextOutlet.stringValue]
let status = SecItemUpdate(updateQuery as CFDictionary, newAttributes as CFDictionary)
per osstatus.com, -50 is "One or more parameters passed to the function were not valid."