diff --git a/dom/events/Event.cpp b/dom/events/Event.cpp
index 0d8d1ea328f2..646dd88ae608 100644
--- a/dom/events/Event.cpp
+++ b/dom/events/Event.cpp
@@ -523,6 +523,7 @@ void
Event::SetEventType(const nsAString& aEventTypeArg)
{
if (mIsMainThreadEvent) {
+ mEvent->typeString.Truncate();
mEvent->userType =
nsContentUtils::GetEventIdAndAtom(aEventTypeArg, mEvent->eventStructType,
&(mEvent->message));
diff --git a/dom/events/test/mochitest.ini b/dom/events/test/mochitest.ini
index 0b790b6e8675..4eeff715106e 100644
--- a/dom/events/test/mochitest.ini
+++ b/dom/events/test/mochitest.ini
@@ -157,3 +157,4 @@ skip-if = toolkit == 'android' #CRASH_DUMP, RANDOM
skip-if = buildapp == 'b2g' || e10s
[test_bug985988.html]
[test_dom_storage_event.html]
+[test_bug998809.html]
diff --git a/dom/events/test/test_bug998809.html b/dom/events/test/test_bug998809.html
new file mode 100644
index 000000000000..08499d2c75af
--- /dev/null
+++ b/dom/events/test/test_bug998809.html
@@ -0,0 +1,35 @@
+
+
+
+
+
+ Test for Bug 998809
+
+
+
+
+
+Mozilla Bug 998809
+
+
+
+
+
+
+
+