invalidSubscriberAttributesError (error code 21): Invalid subscriber attributes

• The backend answered 7263 (invalid subscriber attributes) or 7264 (invalid subscriber attributes body), which both SDKs map here (BackendErrorCode.swift, errors.kt).
• A reserved attribute, one whose key starts with $, has a bad value, for example an email address that is not an email.
• A custom attribute has a key or value the server refuses.
• On iOS, the failing keys and messages come back in the error's attribute_errors entry in userInfo (ErrorDetails.swift).

• Do not retry the same values. They will fail the same way.
• Do not show this to customers. It is a data-quality problem, not something they can fix.
• On iOS, a failure for a custom key (one without $) counts as a blocking error for the combined attributes-and-offerings sync, but a failure for a reserved $ key does not (Purchases.swift). Do not grant or withhold access based on it.

// Swift
do {
  let result = try await Purchases.shared.purchase(package: package)
} catch let error as ErrorCode where error == .invalidSubscriberAttributesError {
  // handle invalid subscriber attributes
}

// Kotlin
if (error.code == PurchasesErrorCode.InvalidSubscriberAttributesError) { /* ... */ }

On RevenueDot: RevenueDot validates $email. If it is not an email address, it answers HTTP 400 with code 7263, lists the key in attribute_errors, and still saves every other attribute in the request.