diff --git a/fpdfsdk/cpdfsdk_customaccess.cpp b/fpdfsdk/cpdfsdk_customaccess.cpp
index ccb87a9..108ffbc 100644
--- a/fpdfsdk/cpdfsdk_customaccess.cpp
+++ b/fpdfsdk/cpdfsdk_customaccess.cpp
@@ -23,7 +23,10 @@
   if (!buffer || offset < 0 || !size)
     return false;
 
-  FX_SAFE_FILESIZE new_pos = pdfium::base::checked_cast<FX_FILESIZE>(size);
+  if (!pdfium::base::IsValueInRangeForNumericType<FX_FILESIZE>(size))
+    return false;
+
+  FX_SAFE_FILESIZE new_pos = size;
   new_pos += offset;
   return new_pos.IsValid() && new_pos.ValueOrDie() <= GetSize() &&
          m_FileAccess.m_GetBlock(m_FileAccess.m_Param, offset,
diff --git a/fpdfsdk/fpdf_dataavail.cpp b/fpdfsdk/fpdf_dataavail.cpp
index 2384b6e..11abff3 100644
--- a/fpdfsdk/fpdf_dataavail.cpp
+++ b/fpdfsdk/fpdf_dataavail.cpp
@@ -78,7 +78,10 @@
     if (!buffer || offset < 0 || !size)
       return false;
 
-    FX_SAFE_FILESIZE new_pos = pdfium::base::checked_cast<FX_FILESIZE>(size);
+    if (!pdfium::base::IsValueInRangeForNumericType<FX_FILESIZE>(size))
+      return false;
+
+    FX_SAFE_FILESIZE new_pos = size;
     new_pos += offset;
     return new_pos.IsValid() && new_pos.ValueOrDie() <= GetSize() &&
            file_->m_GetBlock(file_->m_Param, offset,