diff --git a/testing/marionette/error.js b/testing/marionette/error.js
index 97e3f2183410..ddd81344d961 100644
--- a/testing/marionette/error.js
+++ b/testing/marionette/error.js
@@ -211,7 +211,7 @@ error.fromJson = function (json) {
  *     to propagate.
  */
 this.WebDriverError = function (err) {
-  Error.call(this, err);
+  const proto = Error.call(this, err);
   this.name = "WebDriverError";
   this.status = "webdriver error";
 
@@ -220,6 +220,7 @@ this.WebDriverError = function (err) {
     this.stack = err.stack;
   } else {
     this.message = err;
+    this.stack = proto.stack;
   }
 };
 WebDriverError.prototype = Object.create(Error.prototype);
diff --git a/testing/marionette/test_error.js b/testing/marionette/test_error.js
index acb37dbb59c0..3f36902a46ee 100644
--- a/testing/marionette/test_error.js
+++ b/testing/marionette/test_error.js
@@ -81,12 +81,15 @@ add_test(function test_toJson() {
       /Unserialisable error type: [object Error]/);
 
   let e0 = new WebDriverError();
-  deepEqual({error: e0.status, message: "", stacktrace: ""},
-      error.toJson(e0));
+  let e0s = error.toJson(e0);
+  equal(e0s.error, "webdriver error");
+  equal(e0s.message, "");
+  equal(e0s.stacktrace, e0.stack);
 
   let e1 = new WebDriverError("a");
-  deepEqual({error: e1.status, message: "a", stacktrace: ""},
-      error.toJson(e1));
+  let e1s = error.toJson(e1);
+  equal(e1s.message, e1.message);
+  equal(e1s.stacktrace, e1.stack);
 
   let e2 = new JavaScriptError("first", "second", "third", "fourth");
   let e2s = error.toJson(e2);
@@ -107,10 +110,17 @@ add_test(function test_fromJson() {
   Assert.throws(() => error.fromJson({}),
       /Undeserialisable error type: undefined/);
 
+  // stacks will be different
   let e1 = new WebDriverError("1");
-  deepEqual(e1, error.fromJson({error: "webdriver error", message: "1"}));
+  let e1r = error.fromJson({error: "webdriver error", message: "1"});
+  equal(e1r.status, e1.status);
+  equal(e1r.message, e1.message);
+
+  // stacks will be different
   let e2 = new InvalidArgumentError("2");
-  deepEqual(e2, error.fromJson({error: "invalid argument", message: "2"}));
+  let e2r = error.fromJson({error: "invalid argument", message: "2"});
+  equal(e2r.status, e2.status);
+  equal(e2r.message, e2.message);
 
   let e3 = new JavaScriptError("first", "second", "third", "fourth");
   let e3s = error.toJson(e3);