diff --git a/core/fpdfapi/page/cpdf_page.h b/core/fpdfapi/page/cpdf_page.h
index 3fe4a80..58209b6 100644
--- a/core/fpdfapi/page/cpdf_page.h
+++ b/core/fpdfapi/page/cpdf_page.h
@@ -115,7 +115,7 @@
 
   CFX_SizeF m_PageSize;
   CFX_Matrix m_PageMatrix;
-  UnownedPtr<CPDF_Document> m_pPDFDocument;
+  UnownedPtr<CPDF_Document> const m_pPDFDocument;
   std::unique_ptr<RenderCacheIface> m_pRenderCache;
   std::unique_ptr<RenderContextIface> m_pRenderContext;
   ObservedPtr<View> m_pView;