diff --git a/gfx/layers/ipc/CompositorVsyncScheduler.cpp b/gfx/layers/ipc/CompositorVsyncScheduler.cpp
index 6e956e55f0f6..c3bf8a240c41 100644
--- a/gfx/layers/ipc/CompositorVsyncScheduler.cpp
+++ b/gfx/layers/ipc/CompositorVsyncScheduler.cpp
@@ -334,5 +334,12 @@ CompositorVsyncScheduler::ScheduleTask(already_AddRefed<CancelableRunnable> aTas
   CompositorThreadHolder::Loop()->PostDelayedTask(Move(aTask), 0);
 }
 
+const TimeStamp&
+CompositorVsyncScheduler::GetLastComposeTime() const
+{
+  MOZ_ASSERT(CompositorThreadHolder::IsInCompositorThread());
+  return mLastCompose;
+}
+
 } // namespace layers
 } // namespace mozilla
diff --git a/gfx/layers/ipc/CompositorVsyncScheduler.h b/gfx/layers/ipc/CompositorVsyncScheduler.h
index 79885e70a74e..266d9c81a3a4 100644
--- a/gfx/layers/ipc/CompositorVsyncScheduler.h
+++ b/gfx/layers/ipc/CompositorVsyncScheduler.h
@@ -83,11 +83,11 @@ public:
    */
   void ForceComposeToTarget(gfx::DrawTarget* aTarget, const gfx::IntRect* aRect);
 
-  const TimeStamp& GetLastComposeTime()
-  {
-    return mLastCompose;
-  }
-
+  /**
+   * Return the vsync timestamp of the last or ongoing composite. Must be called
+   * on the compositor thread.
+   */
+  const TimeStamp& GetLastComposeTime() const;
 #ifdef COMPOSITOR_PERFORMANCE_WARNING
   const TimeStamp& GetExpectedComposeStartTime()
   {