diff --git a/CHANGELOG.md b/CHANGELOG.md
index 984253ff..c10c778a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -46,7 +46,7 @@ _None._
 
 ### Internal Changes
 
-_None._
+- Fix `WordPressAPIError`'s localized error message. [#662]
 
 ## 9.0.0
 
diff --git a/WordPressKit.xcodeproj/project.pbxproj b/WordPressKit.xcodeproj/project.pbxproj
index 86714efd..26028053 100644
--- a/WordPressKit.xcodeproj/project.pbxproj
+++ b/WordPressKit.xcodeproj/project.pbxproj
@@ -144,6 +144,7 @@
 		4A11239E2B1926D1004690CF /* HTTPClient.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A11239D2B1926D1004690CF /* HTTPClient.swift */; };
 		4A1DEF44293051BC00322608 /* LoggingTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A1DEF43293051BC00322608 /* LoggingTests.swift */; };
 		4A1DEF46293051C600322608 /* LoggingTests.m in Sources */ = {isa = PBXBuildFile; fileRef = 4A1DEF45293051C600322608 /* LoggingTests.m */; };
+		4A40F6552B2A5A1A0015DA77 /* WordPressAPIErrorTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A40F6542B2A5A1A0015DA77 /* WordPressAPIErrorTests.swift */; };
 		4A68E3CD29404181004AC3DC /* RemoteBlog.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A68E3CC29404181004AC3DC /* RemoteBlog.swift */; };
 		4A68E3CF29404289004AC3DC /* RemoteBlogOptionsHelper.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A68E3CE29404289004AC3DC /* RemoteBlogOptionsHelper.swift */; };
 		4A68E3D329406AA0004AC3DC /* RemoteMenu.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4A68E3D029406AA0004AC3DC /* RemoteMenu.swift */; };
@@ -840,6 +841,7 @@
 		4A11239D2B1926D1004690CF /* HTTPClient.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = HTTPClient.swift; sourceTree = "<group>"; };
 		4A1DEF43293051BC00322608 /* LoggingTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = LoggingTests.swift; sourceTree = "<group>"; };
 		4A1DEF45293051C600322608 /* LoggingTests.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = LoggingTests.m; sourceTree = "<group>"; };
+		4A40F6542B2A5A1A0015DA77 /* WordPressAPIErrorTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = WordPressAPIErrorTests.swift; sourceTree = "<group>"; };
 		4A68E3CC29404181004AC3DC /* RemoteBlog.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RemoteBlog.swift; sourceTree = "<group>"; };
 		4A68E3CE29404289004AC3DC /* RemoteBlogOptionsHelper.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RemoteBlogOptionsHelper.swift; sourceTree = "<group>"; };
 		4A68E3D029406AA0004AC3DC /* RemoteMenu.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RemoteMenu.swift; sourceTree = "<group>"; };
@@ -2638,6 +2640,7 @@
 				4A1DEF45293051C600322608 /* LoggingTests.m */,
 				4A6B4A832B26974F00802316 /* HTTPRequestBuilderTests.swift */,
 				4A6B4A852B269D0C00802316 /* URLSessionHelperTests.swift */,
+				4A40F6542B2A5A1A0015DA77 /* WordPressAPIErrorTests.swift */,
 			);
 			path = Utilities;
 			sourceTree = "<group>";
@@ -3502,6 +3505,7 @@
 				740B23D61F17F7C100067A2A /* XMLRPCTestable.swift in Sources */,
 				FFE247A720C891D1002DF3A2 /* WordPressComOAuthClientTests.swift in Sources */,
 				93AB06041EE8838400EF8764 /* RemoteTestCase.swift in Sources */,
+				4A40F6552B2A5A1A0015DA77 /* WordPressAPIErrorTests.swift in Sources */,
 				BA2A78FA24A486D300BB6F53 /* SitePluginTests.swift in Sources */,
 				465F88A7263B371D00F4C950 /* BlockEditorSettingsServiceRemoteTests.swift in Sources */,
 				93BD27411EE73311002BB00B /* AccountServiceRemoteRESTTests.swift in Sources */,
diff --git a/WordPressKit/WordPressAPIError.swift b/WordPressKit/WordPressAPIError.swift
index fa15b64a..2e2518cd 100644
--- a/WordPressKit/WordPressAPIError.swift
+++ b/WordPressKit/WordPressAPIError.swift
@@ -21,23 +21,30 @@ public enum WordPressAPIError<EndpointError>: Error where EndpointError: Localiz
     case unknown(underlyingError: Error)
 }
 
-extension WordPressComOAuthError: LocalizedError {
+extension WordPressAPIError: LocalizedError {
 
     public var errorDescription: String? {
+        // Considering `WordPressAPIError` is the error that's surfaced from this library to the apps, its instanes
+        // may be displayed on UI directly. To prevent Swift's default error message (i.e. "This operation can't be
+        // completed. <SwiftTypeName> (code=...)") from being displayed, we need to make sure this implementation
+        // always returns a non-nil value.
+        let localizedErrorMessage: String
         switch self {
         case .requestEncodingFailure, .unparsableResponse:
             // These are usually programming errors.
-            return Self.unknownErrorMessage
+            localizedErrorMessage = Self.unknownErrorMessage
         case let .endpointError(error):
-            return error.errorDescription
+            localizedErrorMessage = error.errorDescription ?? Self.unknownErrorMessage
         case let .connection(error):
-            return error.localizedDescription
+            localizedErrorMessage = error.localizedDescription
         case let .unknown(underlyingError):
             if let msg = (underlyingError as? LocalizedError)?.errorDescription {
-                return msg
+                localizedErrorMessage = msg
+            } else {
+                localizedErrorMessage = Self.unknownErrorMessage
             }
-            return Self.unknownErrorMessage
         }
+        return localizedErrorMessage
     }
 
 }
diff --git a/WordPressKitTests/Utilities/WordPressAPIErrorTests.swift b/WordPressKitTests/Utilities/WordPressAPIErrorTests.swift
new file mode 100644
index 00000000..2b653f7d
--- /dev/null
+++ b/WordPressKitTests/Utilities/WordPressAPIErrorTests.swift
@@ -0,0 +1,27 @@
+import Foundation
+import XCTest
+@testable import WordPressKit
+
+class WordPressAPIErrorTests: XCTestCase {
+
+    func testLocalizedMessage() {
+        struct TestError: LocalizedError {
+            var errorDescription: String? = "this is a test error"
+        }
+
+        let error = WordPressAPIError.endpointError(TestError())
+        XCTAssertEqual(error.errorDescription, "this is a test error")
+        XCTAssertEqual((error as NSError).localizedDescription, "this is a test error")
+    }
+
+    func testNilErrorDescription() {
+        struct TestError: LocalizedError {
+            var errorDescription: String? = nil
+        }
+
+        let error = WordPressAPIError.endpointError(TestError())
+        XCTAssertEqual(error.localizedDescription, WordPressAPIError<TestError>.unknownErrorMessage)
+        XCTAssertEqual((error as NSError).localizedDescription, WordPressAPIError<TestError>.unknownErrorMessage)
+    }
+
+}