--- a/testing/modules/Assert.jsm
+++ b/testing/modules/Assert.jsm
@@ -303,17 +303,17 @@ proto.strictEqual = function strictEqual
  * @param message (optional)
  *        (string) Short explanation of the expected result
  */
 proto.notStrictEqual = function notStrictEqual(actual, expected, message) {
   this.report(actual === expected, actual, expected, message, "!==");
 };
 
 function expectedException(actual, expected) {
-  if (!actual || !expected) {
+  if (!expected) {
     return false;
   }
 
   if (instanceOf(expected, "RegExp")) {
     return expected.test(actual);
   // We need to guard against the right hand parameter of "instanceof" lacking
   // the "prototype" property, which is true of arrow functions in particular.
   } else if (!(typeof expected === "function" && !expected.prototype) &&
@@ -353,26 +353,29 @@ function expectedException(actual, expec
 proto.throws = function(block, expected, message) {
   let actual;
 
   if (typeof expected === "string") {
     message = expected;
     expected = null;
   }
 
+  var has_thrown;
   try {
     block();
+    has_thrown = false;
   } catch (e) {
+    has_thrown = true;
     actual = e;
   }
 
   message = (expected && expected.name ? " (" + expected.name + ")." : ".") +
             (message ? " " + message : ".");
 
-  if (!actual) {
+  if (!has_thrown) {
     this.report(true, actual, expected, "Missing expected exception" + message);
   }
 
   if ((actual && expected && !expectedException(actual, expected))) {
     throw actual;
   }
 
   this.report(false, expected, expected, message);