diff --git a/core/fxge/agg/fx_agg_driver.cpp b/core/fxge/agg/fx_agg_driver.cpp
index fd3c75e..70159ea 100644
--- a/core/fxge/agg/fx_agg_driver.cpp
+++ b/core/fxge/agg/fx_agg_driver.cpp
@@ -1382,9 +1382,8 @@
     bool bRgbByteOrder,
     RetainPtr<CFX_DIBitmap> pBackdropBitmap,
     bool bGroupKnockout) {
-  if (!pBitmap)
-    return false;
-
+  // Unlike the Skia version, all callers pass in a non-null `pBitmap`.
+  CHECK(pBitmap);
   SetBitmap(pBitmap);
   SetDeviceDriver(std::make_unique<pdfium::CFX_AggDeviceDriver>(
       std::move(pBitmap), bRgbByteOrder, std::move(pBackdropBitmap),
diff --git a/core/fxge/skia/fx_skia_device.cpp b/core/fxge/skia/fx_skia_device.cpp
index ddd3d44..7c31bcf 100644
--- a/core/fxge/skia/fx_skia_device.cpp
+++ b/core/fxge/skia/fx_skia_device.cpp
@@ -1635,6 +1635,8 @@
     bool bRgbByteOrder,
     RetainPtr<CFX_DIBitmap> pBackdropBitmap,
     bool bGroupKnockout) {
+  // FPDF_FFLDrawSkia() ends up calling this method with a deliberately null
+  // `pBitmap`.
   if (!pBitmap)
     return false;
   SetBitmap(pBitmap);