diff --git a/core/fpdfapi/page/cpdf_allstates.cpp b/core/fpdfapi/page/cpdf_allstates.cpp
index 3312547..5af6327 100644
--- a/core/fpdfapi/page/cpdf_allstates.cpp
+++ b/core/fpdfapi/page/cpdf_allstates.cpp
@@ -42,7 +42,7 @@
   m_GraphState.SetLineDash(std::move(dashes), phase, scale);
 }
 
-void CPDF_AllStates::ProcessExtGS(CPDF_Dictionary* pGS,
+void CPDF_AllStates::ProcessExtGS(const CPDF_Dictionary* pGS,
                                   CPDF_StreamContentParser* pParser) {
   CPDF_DictionaryLocker locker(pGS);
   for (const auto& it : locker) {
diff --git a/core/fpdfapi/page/cpdf_allstates.h b/core/fpdfapi/page/cpdf_allstates.h
index 72e41d3..7586e2a 100644
--- a/core/fpdfapi/page/cpdf_allstates.h
+++ b/core/fpdfapi/page/cpdf_allstates.h
@@ -20,7 +20,8 @@
   ~CPDF_AllStates() override;
 
   void Copy(const CPDF_AllStates& src);
-  void ProcessExtGS(CPDF_Dictionary* pGS, CPDF_StreamContentParser* pParser);
+  void ProcessExtGS(const CPDF_Dictionary* pGS,
+                    CPDF_StreamContentParser* pParser);
   void SetLineDash(const CPDF_Array* pArray, float phase, float scale);
 
   CFX_Matrix m_TextMatrix;