diff --git a/verror/index.d.ts b/verror/index.d.ts
index 35e11f17aa..96f711c9f7 100644
--- a/verror/index.d.ts
+++ b/verror/index.d.ts
@@ -1,16 +1,19 @@
-// Type definitions for verror v1.9.0
+// Type definitions for verror 1.9
 // Project: https://github.com/davepacheco/node-verror
-// Definitions by: Sven Reglitzki <https://github.com/svi3c/>
+// Definitions by: Sven Reglitzki <https://github.com/svi3c/>, Maxime Toumi-M <https://github.com/max4t/>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 
 
-type VErrorInfo = { [key: string]: any };
+// tslint:disable-next-line:functional-interfaces
+interface VErrorInfo {
+    [key: string]: any;
+}
 
 declare interface VErrorOptions {
-    cause: Error;
+    cause?: Error | null | undefined;
     name?: string;
     strict?: boolean;
-    constructorOpt?: Function;
+    constructorOpt?: (...args: any[]) => void;
     info?: VErrorInfo;
 }
 
@@ -70,10 +73,7 @@ declare class MultiError extends VError {
  * is wrapped, not "folded in" as with VError.	Accepts a printf-style message.
  * The cause argument can be null.
  */
-declare class WError extends Error {
-    cause(): Error;
-    constructor(cause: Error, message: string, ...params: any[]);
-    constructor(message: string, ...params: any[]);
+declare class WError extends VError {
 }
 
 export = VError;
diff --git a/verror/tsconfig.json b/verror/tsconfig.json
index bda043af17..1c5ea42467 100644
--- a/verror/tsconfig.json
+++ b/verror/tsconfig.json
@@ -4,7 +4,7 @@
         "target": "es6",
         "noImplicitAny": true,
         "noImplicitThis": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "baseUrl": "../",
         "typeRoots": [
             "../"
@@ -17,4 +17,4 @@
         "index.d.ts",
         "verror-tests.ts"
     ]
-}
\ No newline at end of file
+}
diff --git a/verror/verror-tests.ts b/verror/verror-tests.ts
index 0c386987c1..de1838ee9b 100644
--- a/verror/verror-tests.ts
+++ b/verror/verror-tests.ts
@@ -21,8 +21,10 @@ var verror3 = new VError({
     }
 }, "bar");
 
-var cause1: Error = verror1.cause();
-var cause2: Error = werror.cause();
+var verror4 = new VError({ cause: null }, "bar");
+
+var cause1: Error | undefined = verror1.cause();
+var cause2: Error | undefined = werror.cause();
 
 const info: { [k: string]: any } = VError.info(verror3);
 const namedCause: Error | null = VError.findCauseByName(verror3, "fooError");