diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index d8d66167600..1c0ceb0b808 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -7541,6 +7541,8 @@ int16_t
 nsContentUtils::GetButtonsFlagForButton(int32_t aButton)
 {
   switch (aButton) {
+    case -1:
+      return WidgetMouseEvent::eNoButtonFlag;
     case WidgetMouseEvent::eLeftButton:
       return WidgetMouseEvent::eLeftButtonFlag;
     case WidgetMouseEvent::eMiddleButton:
diff --git a/widget/MouseEvents.h b/widget/MouseEvents.h
index 23f1eb8e170..bdd710ecd0a 100644
--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -111,6 +111,7 @@ public:
   int16_t button;
 
   enum buttonsFlag {
+    eNoButtonFlag     = 0x00,
     eLeftButtonFlag   = 0x01,
     eRightButtonFlag  = 0x02,
     eMiddleButtonFlag = 0x04,