diff --git a/core/fpdfapi/fpdf_page/cpdf_colorstate.h b/core/fpdfapi/fpdf_page/cpdf_colorstate.h
index d928dbd..a4077d9 100644
--- a/core/fpdfapi/fpdf_page/cpdf_colorstate.h
+++ b/core/fpdfapi/fpdf_page/cpdf_colorstate.h
@@ -51,7 +51,7 @@
   // TODO(tsepez): Stop leaking ColorStateData outside this class.
   const CPDF_ColorStateData* GetObject() const { return m_Ref.GetObject(); }
 
-  operator bool() const { return !!m_Ref; }
+  explicit operator bool() const { return !!m_Ref; }
 
  private:
   void SetColor(CPDF_Color& color,
diff --git a/core/fpdfapi/fpdf_page/cpdf_contentmark.h b/core/fpdfapi/fpdf_page/cpdf_contentmark.h
index 2941c61..25da588 100644
--- a/core/fpdfapi/fpdf_page/cpdf_contentmark.h
+++ b/core/fpdfapi/fpdf_page/cpdf_contentmark.h
@@ -30,7 +30,7 @@
                FX_BOOL bDirect);
   void DeleteLastMark();
 
-  operator bool() const { return !!m_Ref; }
+  explicit operator bool() const { return !!m_Ref; }
 
  private:
   CFX_CountRef<CPDF_ContentMarkData> m_Ref;
diff --git a/core/fpdfapi/fpdf_page/include/cpdf_generalstate.h b/core/fpdfapi/fpdf_page/include/cpdf_generalstate.h
index f07e411..1cd2d81 100644
--- a/core/fpdfapi/fpdf_page/include/cpdf_generalstate.h
+++ b/core/fpdfapi/fpdf_page/include/cpdf_generalstate.h
@@ -21,7 +21,7 @@
   ~CPDF_GeneralState();
 
   void Emplace() { m_Ref.Emplace(); }
-  operator bool() const { return !!m_Ref; }
+  explicit operator bool() const { return !!m_Ref; }
 
   void SetRenderIntent(const CFX_ByteString& ri);
 
diff --git a/core/fpdfapi/fpdf_page/include/cpdf_path.h b/core/fpdfapi/fpdf_page/include/cpdf_path.h
index a9b0a7f..ae98531 100644
--- a/core/fpdfapi/fpdf_page/include/cpdf_path.h
+++ b/core/fpdfapi/fpdf_page/include/cpdf_path.h
@@ -20,7 +20,7 @@
   ~CPDF_Path();
 
   void Emplace() { m_Ref.Emplace(); }
-  operator bool() const { return !!m_Ref; }
+  explicit operator bool() const { return !!m_Ref; }
 
   int GetPointCount() const;
   void SetPointCount(int count);