diff --git a/widget/InputData.cpp b/widget/InputData.cpp
index c2edb9ab4346..a10a08d79579 100644
--- a/widget/InputData.cpp
+++ b/widget/InputData.cpp
@@ -382,6 +382,7 @@ ScrollWheelInput::ScrollWheelInput(const WidgetWheelEvent& aWheelEvent)
   , mDeltaY(aWheelEvent.deltaY)
   , mLineOrPageDeltaX(aWheelEvent.lineOrPageDeltaX)
   , mLineOrPageDeltaY(aWheelEvent.lineOrPageDeltaY)
+  , mScrollSeriesNumber(0)
   , mUserDeltaMultiplierX(1.0)
   , mUserDeltaMultiplierY(1.0)
   , mMayHaveMomentum(aWheelEvent.mayHaveMomentum)