diff --git a/Sources/mas/AppStore/ISStoreAccount.swift b/Sources/mas/AppStore/ISStoreAccount.swift
index ecb3313..56b7bd6 100644
--- a/Sources/mas/AppStore/ISStoreAccount.swift
+++ b/Sources/mas/AppStore/ISStoreAccount.swift
@@ -32,7 +32,7 @@ extension ISStoreAccount: StoreAccount {
         return .value(CKAccountStore.shared().primaryAccount)
     }
 
-    static func signIn(username: String, password: String, systemDialog: Bool) -> Promise<ISStoreAccount> {
+    static func signIn(appleID: String, password: String, systemDialog: Bool) -> Promise<ISStoreAccount> {
         // swift-format-ignore: UseEarlyExits
         if #available(macOS 10.13, *) {
             // Signing in is no longer possible as of High Sierra.
@@ -44,7 +44,7 @@ extension ISStoreAccount: StoreAccount {
                 primaryAccount
                 .then { account -> Promise<ISStoreAccount> in
                     if account.isSignedIn {
-                        return Promise(error: MASError.alreadySignedIn(asAccountId: account.identifier))
+                        return Promise(error: MASError.alreadySignedIn(asAppleID: account.identifier))
                     }
 
                     let password =
@@ -57,7 +57,7 @@ extension ISStoreAccount: StoreAccount {
                     }
 
                     let context = ISAuthenticationContext(accountID: 0)
-                    context.appleIDOverride = username
+                    context.appleIDOverride = appleID
 
                     let signInPromise =
                         Promise<ISStoreAccount> { seal in
@@ -77,7 +77,7 @@ extension ISStoreAccount: StoreAccount {
                     }
 
                     context.demoMode = true
-                    context.demoAccountName = username
+                    context.demoAccountName = appleID
                     context.demoAccountPassword = password
                     context.demoAutologinMode = true
 
diff --git a/Sources/mas/Commands/SignIn.swift b/Sources/mas/Commands/SignIn.swift
index ea332a3..aaa4a96 100644
--- a/Sources/mas/Commands/SignIn.swift
+++ b/Sources/mas/Commands/SignIn.swift
@@ -19,14 +19,14 @@ extension Mas {
         @Flag(help: "Complete login with graphical dialog")
         var dialog = false
         @Argument(help: "Apple ID")
-        var username: String
+        var appleID: String
         @Argument(help: "Password")
         var password: String = ""
 
         /// Runs the command.
         func run() throws {
             do {
-                _ = try ISStoreAccount.signIn(username: username, password: password, systemDialog: dialog).wait()
+                _ = try ISStoreAccount.signIn(appleID: appleID, password: password, systemDialog: dialog).wait()
             } catch {
                 throw error as? MASError ?? MASError.signInFailed(error: error as NSError)
             }
diff --git a/Sources/mas/Errors/MASError.swift b/Sources/mas/Errors/MASError.swift
index 7aea57f..1d31b7f 100644
--- a/Sources/mas/Errors/MASError.swift
+++ b/Sources/mas/Errors/MASError.swift
@@ -18,7 +18,7 @@ enum MASError: Error, Equatable {
     case notSignedIn
     case noPasswordProvided
     case signInFailed(error: NSError?)
-    case alreadySignedIn(asAccountId: String)
+    case alreadySignedIn(asAppleID: String)
 
     case purchaseFailed(error: NSError?)
     case downloadFailed(error: NSError?)
@@ -63,8 +63,8 @@ extension MASError: CustomStringConvertible {
                 return "Sign in failed: \(error.localizedDescription)"
             }
             return "Sign in failed"
-        case .alreadySignedIn(let accountId):
-            return "Already signed in as \(accountId)"
+        case .alreadySignedIn(let appleID):
+            return "Already signed in as \(appleID)"
         case .purchaseFailed(let error):
             if let error {
                 return "Download request failed: \(error.localizedDescription)"
diff --git a/Tests/masTests/Errors/MASErrorTestCase.swift b/Tests/masTests/Errors/MASErrorTestCase.swift
index 47c83d6..15cecfc 100644
--- a/Tests/masTests/Errors/MASErrorTestCase.swift
+++ b/Tests/masTests/Errors/MASErrorTestCase.swift
@@ -59,7 +59,7 @@ class MASErrorTestCase: XCTestCase {
     }
 
     func testAlreadySignedIn() {
-        error = .alreadySignedIn(asAccountId: "person@example.com")
+        error = .alreadySignedIn(asAppleID: "person@example.com")
         XCTAssertEqual(error.description, "Already signed in as person@example.com")
     }