--- a/gfx/layers/wr/WebRenderBridgeParent.cpp
+++ b/gfx/layers/wr/WebRenderBridgeParent.cpp
@@ -1194,16 +1194,21 @@ WebRenderBridgeParent::SampleAnimations(
       }
     }
   }
 }
 
 void
 WebRenderBridgeParent::CompositeToTarget(gfx::DrawTarget* aTarget, const gfx::IntRect* aRect)
 {
+  // The two arguments are part of the CompositorVsyncSchedulerOwner API but in
+  // this implementation they should never be non-null.
+  MOZ_ASSERT(aTarget == nullptr);
+  MOZ_ASSERT(aRect == nullptr);
+
   AUTO_PROFILER_TRACING("Paint", "CompositeToTraget");
   if (mPaused) {
     return;
   }
 
   if (!mForceRendering &&
       wr::RenderThread::Get()->TooManyPendingFrames(mApi->GetId())) {
     // Render thread is busy, try next time.