diff --git a/src/vs/base/common/htmlContent.ts b/src/vs/base/common/htmlContent.ts
index ab7ae6065f19d6081b7156747950e27c2367f172..ce013b916cc6e5d945f95989a7c6bcd5f6261d6f 100644
--- a/src/vs/base/common/htmlContent.ts
+++ b/src/vs/base/common/htmlContent.ts
@@ -56,7 +56,7 @@ export function isEmptyMarkdownString(oneOrMany: IMarkdownString | IMarkdownStri
 export function isMarkdownString(thing: any): thing is IMarkdownString {
 	if (thing instanceof MarkdownString) {
 		return true;
-	} else if (typeof thing === 'object') {
+	} else if (thing && typeof thing === 'object') {
 		return typeof (<IMarkdownString>thing).value === 'string'
 			&& (typeof (<IMarkdownString>thing).isTrusted === 'boolean' || (<IMarkdownString>thing).isTrusted === void 0);
 	}
diff --git a/src/vs/workbench/api/node/extHostTypeConverters.ts b/src/vs/workbench/api/node/extHostTypeConverters.ts
index aa6a0d2b7d5eb6483800248f4224477e26a2f2b8..d8aef198fd88f9b197432241a55ee35ea6737beb 100644
--- a/src/vs/workbench/api/node/extHostTypeConverters.ts
+++ b/src/vs/workbench/api/node/extHostTypeConverters.ts
@@ -158,7 +158,7 @@ export namespace MarkdownString {
 	}
 
 	function isCodeblock(thing: any): thing is Codeblock {
-		return typeof thing === 'object'
+		return thing && typeof thing === 'object'
 			&& typeof (<Codeblock>thing).language === 'string'
 			&& typeof (<Codeblock>thing).value === 'string';
 	}