diff --git a/src/vs/platform/files/common/files.ts b/src/vs/platform/files/common/files.ts
index 28792d1ccce..1d5c80bcb83 100644
--- a/src/vs/platform/files/common/files.ts
+++ b/src/vs/platform/files/common/files.ts
@@ -285,30 +285,31 @@ export enum FileSystemProviderErrorCode {
 	FileNotADirectory = 'EntryNotADirectory',
 	FileIsADirectory = 'EntryIsADirectory',
 	NoPermissions = 'NoPermissions',
-	Unavailable = 'Unavailable'
+	Unavailable = 'Unavailable',
+	Unknown = 'Unknown'
 }
 
 export class FileSystemProviderError extends Error {
 
-	constructor(message: string, public readonly code?: FileSystemProviderErrorCode) {
+	constructor(message: string, public readonly code: FileSystemProviderErrorCode) {
 		super(message);
 	}
 }
 
-export function createFileSystemProviderError(error: Error, code?: FileSystemProviderErrorCode): FileSystemProviderError {
+export function createFileSystemProviderError(error: Error, code: FileSystemProviderErrorCode): FileSystemProviderError {
 	const providerError = new FileSystemProviderError(error.toString(), code);
-	markAsFileSystemProviderError(providerError);
+	markAsFileSystemProviderError(providerError, code);
 
 	return providerError;
 }
 
-export function markAsFileSystemProviderError(error: Error, code?: FileSystemProviderErrorCode): Error {
+export function markAsFileSystemProviderError(error: Error, code: FileSystemProviderErrorCode): Error {
 	error.name = code ? `${code} (FileSystemError)` : `FileSystemError`;
 
 	return error;
 }
 
-export function toFileSystemProviderErrorCode(error: Error): FileSystemProviderErrorCode | undefined {
+export function toFileSystemProviderErrorCode(error: Error): FileSystemProviderErrorCode {
 
 	// FileSystemProviderError comes with the code
 	if (error instanceof FileSystemProviderError) {
@@ -319,7 +320,7 @@ export function toFileSystemProviderErrorCode(error: Error): FileSystemProviderE
 	// went through the markAsFileSystemProviderError() method
 	const match = /^(.+) \(FileSystemError\)$/.exec(error.name);
 	if (!match) {
-		return undefined;
+		return FileSystemProviderErrorCode.Unknown;
 	}
 
 	switch (match[1]) {
@@ -331,7 +332,7 @@ export function toFileSystemProviderErrorCode(error: Error): FileSystemProviderE
 		case FileSystemProviderErrorCode.Unavailable: return FileSystemProviderErrorCode.Unavailable;
 	}
 
-	return undefined;
+	return FileSystemProviderErrorCode.Unknown;
 }
 
 export function toFileOperationResult(error: Error): FileOperationResult {
diff --git a/src/vs/workbench/api/node/extHostTypes.ts b/src/vs/workbench/api/node/extHostTypes.ts
index 756d945c972..b173994f35b 100644
--- a/src/vs/workbench/api/node/extHostTypes.ts
+++ b/src/vs/workbench/api/node/extHostTypes.ts
@@ -2206,12 +2206,12 @@ export class FileSystemError extends Error {
 		return new FileSystemError(messageOrUri, FileSystemProviderErrorCode.Unavailable, FileSystemError.Unavailable);
 	}
 
-	constructor(uriOrMessage?: string | URI, code?: string, terminator?: Function) {
+	constructor(uriOrMessage?: string | URI, code: FileSystemProviderErrorCode = FileSystemProviderErrorCode.Unknown, terminator?: Function) {
 		super(URI.isUri(uriOrMessage) ? uriOrMessage.toString(true) : uriOrMessage);
 
 		// mark the error as file system provider error so that
 		// we can extract the error code on the receiving side
-		markAsFileSystemProviderError(this);
+		markAsFileSystemProviderError(this, code);
 
 		// workaround when extending builtin objects and when compiling to ES5, see:
 		// https://github.com/Microsoft/TypeScript-wiki/blob/master/Breaking-Changes.md#extending-built-ins-like-error-array-and-map-may-no-longer-work
diff --git a/src/vs/workbench/services/files2/node/diskFileSystemProvider.ts b/src/vs/workbench/services/files2/node/diskFileSystemProvider.ts
index dca3f2a9173..cfe5c21add9 100644
--- a/src/vs/workbench/services/files2/node/diskFileSystemProvider.ts
+++ b/src/vs/workbench/services/files2/node/diskFileSystemProvider.ts
@@ -300,7 +300,7 @@ export class DiskFileSystemProvider extends Disposable implements IFileSystemPro
 			return error; // avoid double conversion
 		}
 
-		let code: FileSystemProviderErrorCode | undefined = undefined;
+		let code: FileSystemProviderErrorCode;
 		switch (error.code) {
 			case 'ENOENT':
 				code = FileSystemProviderErrorCode.FileNotFound;
@@ -315,6 +315,8 @@ export class DiskFileSystemProvider extends Disposable implements IFileSystemPro
 			case 'EACCESS':
 				code = FileSystemProviderErrorCode.NoPermissions;
 				break;
+			default:
+				code = FileSystemProviderErrorCode.Unknown;
 		}
 
 		return createFileSystemProviderError(error, code);