--- a/testing/marionette/error.js
+++ b/testing/marionette/error.js
@@ -135,18 +135,18 @@ error.stringify = function(err) {
  */
 error.toJson = function(err) {
   if (!error.isWebDriverError(err)) {
     throw new TypeError(`Unserialisable error type: ${err}`);
   }
 
   let json = {
     error: err.status,
-    message: err.message || null,
-    stacktrace: err.stack || null,
+    message: err.message || "",
+    stacktrace: err.stack || "",
   };
   return json;
 };
 
 /**
  * Unmarshal a JSON dictionary to a WebDriverError prototype.
  *
  * @param {Object.<string, string>} json

--- a/testing/marionette/test_error.js
+++ b/testing/marionette/test_error.js
@@ -88,18 +88,22 @@ add_test(function test_stringify() {
 
   run_next_test();
 });
 
 add_test(function test_toJson() {
   Assert.throws(() => error.toJson(new Error()),
       /Unserialisable error type: [object Error]/);
 
+  let e0 = new WebDriverError();
+  deepEqual({error: e0.status, message: "", stacktrace: ""},
+      error.toJson(e0));
+
   let e1 = new WebDriverError("a");
-  deepEqual({error: e1.status, message: "a", stacktrace: null},
+  deepEqual({error: e1.status, message: "a", stacktrace: ""},
       error.toJson(e1));
 
   let e2 = new JavaScriptError("first", "second", "third", "fourth");
   let e2s = error.toJson(e2);
   equal(e2.status, e2s.error);
   equal(e2.message, e2s.message);
   ok(e2s.stacktrace.match(/second/));
   ok(e2s.stacktrace.match(/third/));