diff --git a/src/vs/workbench/api/common/extHostTreeViews.ts b/src/vs/workbench/api/common/extHostTreeViews.ts
index 1165641848c..537c3e15849 100644
--- a/src/vs/workbench/api/common/extHostTreeViews.ts
+++ b/src/vs/workbench/api/common/extHostTreeViews.ts
@@ -100,7 +100,10 @@ export class ExtHostTreeViews implements ExtHostTreeViewsShape {
 			get onDidChangeSelection() { return treeView.onDidChangeSelection; },
 			get visible() { return treeView.visible; },
 			get onDidChangeVisibility() { return treeView.onDidChangeVisibility; },
-			get onDidChangeCheckboxState() { checkProposedApiEnabled(extension, 'treeItemCheckbox'); return treeView.onDidChangeCheckboxState; },
+			get onDidChangeCheckboxState() {
+				checkProposedApiEnabled(extension, 'treeItemCheckbox');
+				return treeView.onDidChangeCheckboxState;
+			},
 			get message() { return treeView.message; },
 			set message(message: string) {
 				treeView.message = message;
@@ -488,6 +491,7 @@ class ExtHostTreeView<T> extends Disposable {
 	}
 
 	async setCheckboxState(checkboxUpdates: CheckboxUpdate[]) {
+		type CheckboxUpdateWithItem = { extensionItem: NonNullable<T>; treeItem: vscode.TreeItem2; newState: TreeItemCheckboxState };
 		const items = (await Promise.all(checkboxUpdates.map(async checkboxUpdate => {
 			const extensionItem = this.getExtensionElement(checkboxUpdate.treeItemHandle);
 			if (extensionItem) {
@@ -498,7 +502,7 @@ class ExtHostTreeView<T> extends Disposable {
 				};
 			}
 			return Promise.resolve(undefined);
-		}))).filter((item) => item !== undefined) as { extensionItem: T; treeItem: vscode.TreeItem2; newState: TreeItemCheckboxState }[];
+		}))).filter<CheckboxUpdateWithItem>((item): item is CheckboxUpdateWithItem => item !== undefined);
 
 		items.forEach(item => {
 			item.treeItem.checkboxState = item.newState ? TreeItemCheckboxState.Checked : TreeItemCheckboxState.Unchecked;