diff --git a/fpdfsdk/pwl/cpwl_combo_box.cpp b/fpdfsdk/pwl/cpwl_combo_box.cpp
index 8c43da7..37127e7 100644
--- a/fpdfsdk/pwl/cpwl_combo_box.cpp
+++ b/fpdfsdk/pwl/cpwl_combo_box.cpp
@@ -461,6 +461,7 @@
 }
 
 void CPWL_ComboBox::SetFillerNotify(IPWL_FillerNotify* pNotify) {
+  CHECK_EQ(pNotify, GetCreationParams()->pFillerNotify);
   m_pFillerNotify = pNotify;
 
   if (m_pEdit)
diff --git a/fpdfsdk/pwl/cpwl_edit.h b/fpdfsdk/pwl/cpwl_edit.h
index 0656088..845f338 100644
--- a/fpdfsdk/pwl/cpwl_edit.h
+++ b/fpdfsdk/pwl/cpwl_edit.h
@@ -16,6 +16,7 @@
 #include "core/fxcrt/widestring.h"
 #include "fpdfsdk/pwl/cpwl_wnd.h"
 #include "fpdfsdk/pwl/ipwl_fillernotify.h"
+#include "third_party/base/check_op.h"
 
 class CPDF_Font;
 class CPWL_Caret;
@@ -96,6 +97,7 @@
                                         int32_t nCharArray);
 
   void SetFillerNotify(IPWL_FillerNotify* pNotify) {
+    CHECK_EQ(pNotify, GetCreationParams()->pFillerNotify);
     m_pFillerNotify = pNotify;
   }
 
diff --git a/fpdfsdk/pwl/cpwl_list_box.h b/fpdfsdk/pwl/cpwl_list_box.h
index d72672a..f0df413 100644
--- a/fpdfsdk/pwl/cpwl_list_box.h
+++ b/fpdfsdk/pwl/cpwl_list_box.h
@@ -12,6 +12,7 @@
 #include "core/fxcrt/unowned_ptr.h"
 #include "fpdfsdk/pwl/cpwl_list_ctrl.h"
 #include "fpdfsdk/pwl/cpwl_wnd.h"
+#include "third_party/base/check_op.h"
 
 class IPWL_FillerNotify;
 
@@ -76,6 +77,7 @@
   CFX_FloatRect GetListRect() const;
 
   void SetFillerNotify(IPWL_FillerNotify* pNotify) {
+    CHECK_EQ(pNotify, GetCreationParams()->pFillerNotify);
     m_pFillerNotify = pNotify;
   }