diff --git a/browser/components/loop/content/libs/l10n.js b/browser/components/loop/content/libs/l10n.js
index 8429fcfaaff0..f33bac91cbbf 100644
--- a/browser/components/loop/content/libs/l10n.js
+++ b/browser/components/loop/content/libs/l10n.js
@@ -38,8 +38,9 @@
 
   // translate a string
   function translateString(key, args, fallback) {
-    if (args && args.num) {
-      var num = args && args.num;
+    var num;
+    if (args && ("num" in args)) {
+      num = args.num;
     }
     var data = getL10nData(key, num);
     if (!data && fallback)
diff --git a/browser/components/loop/test/desktop-local/l10n_test.js b/browser/components/loop/test/desktop-local/l10n_test.js
index f76b0a19773e..0f15dbacd0bb 100644
--- a/browser/components/loop/test/desktop-local/l10n_test.js
+++ b/browser/components/loop/test/desktop-local/l10n_test.js
@@ -34,4 +34,8 @@ describe("document.mozL10n", function() {
   it("should get a plural form", function() {
     expect(document.mozL10n.get("plural", {num: 10})).eql("10 plural forms");
   });
+
+  it("should correctly get a plural form for num = 0", function() {
+    expect(document.mozL10n.get("plural", {num: 0})).eql("0 plural form");
+  });
 });