--- a/widget/gtk/WindowSurfaceWayland.cpp
+++ b/widget/gtk/WindowSurfaceWayland.cpp
@@ -418,17 +418,17 @@ WaylandShmPool::Resize(int aSize)
   mAllocatedSize = aSize;
   return true;
 }
 
 void
 WaylandShmPool::SetImageDataFromPool(class WaylandShmPool* aSourcePool,
                                      int aImageDataSize)
 {
-  MOZ_ASSERT(mAllocatedSize <= aImageDataSize, "WaylandShmPool overflows!");
+  MOZ_ASSERT(mAllocatedSize >= aImageDataSize, "WaylandShmPool overflows!");
   memcpy(mImageData, aSourcePool->GetImageData(), aImageDataSize);
 }
 
 WaylandShmPool::~WaylandShmPool()
 {
   munmap(mImageData, mAllocatedSize);
   wl_shm_pool_destroy(mShmPool);
   close(mShmPoolFd);