--- a/widget/windows/PDFViaEMFPrintHelper.h
+++ b/widget/windows/PDFViaEMFPrintHelper.h
@@ -32,17 +32,17 @@ public:
   ~PDFViaEMFPrintHelper();
 
   /** Loads the specified PDF file. */
   NS_IMETHOD OpenDocument(nsIFile *aFile);
 
   /** Releases document buffer. */
   void CloseDocument();
 
-  int GetPageCount() { return mPDFiumEngine->GetPageCount(mPDFDoc); }
+  int GetPageCount() const { return mPDFiumEngine->GetPageCount(mPDFDoc); }
 
   /** Convert specified PDF page to EMF and draw the EMF onto the given DC. */
   bool DrawPage(HDC aPrinterDC, unsigned int aPageIndex,
                 int aPageWidth, int aPageHeight);
 
   /** Convert specified PDF page to EMF and save it to file. */
   bool DrawPageToFile(const wchar_t* aFilePath, unsigned int aPageIndex,
                       int aPageWidth, int aPageHeight);