diff --git a/src/vs/workbench/api/node/extHostTreeViews.ts b/src/vs/workbench/api/node/extHostTreeViews.ts
index a96594f0e3a..0f7194fcc70 100644
--- a/src/vs/workbench/api/node/extHostTreeViews.ts
+++ b/src/vs/workbench/api/node/extHostTreeViews.ts
@@ -78,6 +78,7 @@ interface TreeNode {
 class ExtHostTreeView<T> extends Disposable {
 
 	private static LABEL_HANDLE_PREFIX = '0';
+	private static ID_HANDLE_PREFIX = '1';
 
 	private rootHandles: TreeItemHandle[] = [];
 	private elements: Map<TreeItemHandle, T> = new Map<TreeItemHandle, T>();
@@ -105,6 +106,9 @@ class ExtHostTreeView<T> extends Disposable {
 					asWinJsPromise(() => this.dataProvider.getTreeItem(element))
 						.then(extTreeItem => {
 							if (extTreeItem) {
+								if (typeof element === 'string' && this.elements.has(this.createHandle(element, extTreeItem))) {
+									throw new Error(localize('treeView.duplicateElement', 'Element {0} is already registered', element));
+								}
 								return { element, extTreeItem };
 							}
 							return null;
@@ -194,6 +198,10 @@ class ExtHostTreeView<T> extends Disposable {
 	}
 
 	private createHandle(element: T, { label }: vscode.TreeItem, parentHandle?: TreeItemHandle): TreeItemHandle {
+		if (typeof element === 'string') {
+			return `${ExtHostTreeView.ID_HANDLE_PREFIX}/${element}`;
+		}
+
 		const prefix = parentHandle ? parentHandle : ExtHostTreeView.LABEL_HANDLE_PREFIX;
 		label = label.indexOf('/') !== -1 ? label.replace('/', '//') : label;
 		const existingHandle = this.nodes.has(element) ? this.nodes.get(element).handle : void 0;
diff --git a/src/vs/workbench/test/electron-browser/api/extHostTreeViews.test.ts b/src/vs/workbench/test/electron-browser/api/extHostTreeViews.test.ts
index c5a1d5824b6..8cf64585e53 100644
--- a/src/vs/workbench/test/electron-browser/api/extHostTreeViews.test.ts
+++ b/src/vs/workbench/test/electron-browser/api/extHostTreeViews.test.ts
@@ -111,24 +111,24 @@ suite('ExtHostTreeView', function () {
 		return testObject.$getElements('testStringTreeProvider')
 			.then(elements => {
 				const actuals = elements.map(e => e.handle);
-				assert.deepEqual(actuals, ['0/0:a', '0/0:b']);
+				assert.deepEqual(actuals, ['1/a', '1/b']);
 				return TPromise.join([
-					testObject.$getChildren('testStringTreeProvider', '0/0:a')
+					testObject.$getChildren('testStringTreeProvider', '1/a')
 						.then(children => {
 							const actuals = children.map(e => e.handle);
-							assert.deepEqual(actuals, ['0/0:a/0:aa', '0/0:a/0:ab']);
+							assert.deepEqual(actuals, ['1/aa', '1/ab']);
 							return TPromise.join([
-								testObject.$getChildren('testStringTreeProvider', '0/0:a/0:aa').then(children => assert.equal(children.length, 0)),
-								testObject.$getChildren('testStringTreeProvider', '0/0:a/0:ab').then(children => assert.equal(children.length, 0))
+								testObject.$getChildren('testStringTreeProvider', '1/aa').then(children => assert.equal(children.length, 0)),
+								testObject.$getChildren('testStringTreeProvider', '1/ab').then(children => assert.equal(children.length, 0))
 							]);
 						}),
-					testObject.$getChildren('testStringTreeProvider', '0/0:b')
+					testObject.$getChildren('testStringTreeProvider', '1/b')
 						.then(children => {
 							const actuals = children.map(e => e.handle);
-							assert.deepEqual(actuals, ['0/0:b/0:ba', '0/0:b/0:bb']);
+							assert.deepEqual(actuals, ['1/ba', '1/bb']);
 							return TPromise.join([
-								testObject.$getChildren('testStringTreeProvider', '0/0:b/0:ba').then(children => assert.equal(children.length, 0)),
-								testObject.$getChildren('testStringTreeProvider', '0/0:b/0:bb').then(children => assert.equal(children.length, 0))
+								testObject.$getChildren('testStringTreeProvider', '1/ba').then(children => assert.equal(children.length, 0)),
+								testObject.$getChildren('testStringTreeProvider', '1/bb').then(children => assert.equal(children.length, 0))
 							]);
 						})
 				]);