diff --git a/src/vs/workbench/api/electron-browser/mainThreadEditors.ts b/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
index d1d7e1cd812..97ff3f6f4a6 100644
--- a/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
+++ b/src/vs/workbench/api/electron-browser/mainThreadEditors.ts
@@ -114,7 +114,7 @@ export class MainThreadTextEditors implements MainThreadTextEditorsShape {
 
 	// --- from extension host process
 
-	$tryShowTextDocument(resource: UriComponents, options: ITextDocumentShowOptions): Promise<string> {
+	$tryShowTextDocument(resource: UriComponents, options: ITextDocumentShowOptions): Promise<string | undefined> {
 		const uri = URI.revive(resource);
 
 		const editorOptions: ITextEditorOptions = {
diff --git a/src/vs/workbench/api/node/extHost.api.impl.ts b/src/vs/workbench/api/node/extHost.api.impl.ts
index 4d2e7c01c30..652f64d8ca2 100644
--- a/src/vs/workbench/api/node/extHost.api.impl.ts
+++ b/src/vs/workbench/api/node/extHost.api.impl.ts
@@ -527,7 +527,7 @@ export function createApiFactory(
 			onDidChangeWorkspaceFolders: function (listener, thisArgs?, disposables?) {
 				return extHostWorkspace.onDidChangeWorkspace(listener, thisArgs, disposables);
 			},
-			asRelativePath: (pathOrUri, includeWorkspace) => {
+			asRelativePath: (pathOrUri, includeWorkspace?) => {
 				return extHostWorkspace.getRelativePath(pathOrUri, includeWorkspace);
 			},
 			findFiles: (include, exclude, maxResults?, token?) => {
diff --git a/src/vs/workbench/api/node/extHost.protocol.ts b/src/vs/workbench/api/node/extHost.protocol.ts
index 0d20d99f8e1..6296303e877 100644
--- a/src/vs/workbench/api/node/extHost.protocol.ts
+++ b/src/vs/workbench/api/node/extHost.protocol.ts
@@ -224,7 +224,7 @@ export interface ITextDocumentShowOptions {
 }
 
 export interface MainThreadTextEditorsShape extends IDisposable {
-	$tryShowTextDocument(resource: UriComponents, options: ITextDocumentShowOptions): Promise<string>;
+	$tryShowTextDocument(resource: UriComponents, options: ITextDocumentShowOptions): Promise<string | undefined>;
 	$registerTextEditorDecorationType(key: string, options: editorCommon.IDecorationRenderOptions): void;
 	$removeTextEditorDecorationType(key: string): void;
 	$tryShowEditor(id: string, position: EditorViewColumn): Promise<void>;
diff --git a/src/vs/workbench/api/node/extHostTextEditors.ts b/src/vs/workbench/api/node/extHostTextEditors.ts
index a42ec9c245f..ce6b21dcc14 100644
--- a/src/vs/workbench/api/node/extHostTextEditors.ts
+++ b/src/vs/workbench/api/node/extHostTextEditors.ts
@@ -75,7 +75,7 @@ export class ExtHostEditors implements ExtHostEditorsShape {
 		}
 
 		return this._proxy.$tryShowTextDocument(document.uri, options).then(id => {
-			const editor = this._extHostDocumentsAndEditors.getEditor(id);
+			const editor = id && this._extHostDocumentsAndEditors.getEditor(id);
 			if (editor) {
 				return editor;
 			} else {
diff --git a/src/vs/workbench/api/node/extHostWorkspace.ts b/src/vs/workbench/api/node/extHostWorkspace.ts
index bcd01710615..ad7f3b3097a 100644
--- a/src/vs/workbench/api/node/extHostWorkspace.ts
+++ b/src/vs/workbench/api/node/extHostWorkspace.ts
@@ -321,10 +321,10 @@ export class ExtHostWorkspace implements ExtHostWorkspaceShape, IExtHostWorkspac
 		return folders[0].uri.fsPath;
 	}
 
-	getRelativePath(pathOrUri: string | vscode.Uri, includeWorkspace?: boolean): string | undefined {
+	getRelativePath(pathOrUri: string | vscode.Uri, includeWorkspace?: boolean): string {
 
 		let resource: URI | undefined;
-		let path: string | undefined;
+		let path: string = '';
 		if (typeof pathOrUri === 'string') {
 			resource = URI.file(pathOrUri);
 			path = pathOrUri;
@@ -354,7 +354,7 @@ export class ExtHostWorkspace implements ExtHostWorkspaceShape, IExtHostWorkspac
 		if (includeWorkspace && folder.name) {
 			result = `${folder.name}/${result}`;
 		}
-		return result;
+		return result!;
 	}
 
 	private trySetWorkspaceFolders(folders: vscode.WorkspaceFolder[]): void {