diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index e6a5897675f..2dc58181a4d 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1276,7 +1276,7 @@ export class MarkdownString {
 		if (thing instanceof MarkdownString) {
 			return true;
 		}
-		return (thing !== undefined) && thing.appendCodeblock && thing.appendMarkdown && thing.appendText && (thing.value !== undefined);
+		return thing && thing.appendCodeblock && thing.appendMarkdown && thing.appendText && (thing.value !== undefined);
 	}
 }