--- a/widget/windows/WindowsEMF.h
+++ b/widget/windows/WindowsEMF.h
@@ -45,17 +45,23 @@ public:
 
   /**
    * If this object was initiaziled using InitForDrawing() then this function
    * returns an HDC that can be drawn to generate the EMF output. Otherwise it
    * returns null. After finishing with the HDC, consumers could call Playback()
    * to draw EMF onto the given DC or call SaveToFile() to finish writing the
    * EMF file.
    */
-  HDC GetDC() { return mDC; }
+  HDC GetDC() const
+  {
+    MOZ_ASSERT(mDC, "GetDC can be used only after "
+                    "InitForDrawing/ InitFromFileContents and before"
+                    "Playback/ SaveToFile");
+    return mDC;
+  }
 
   /**
    * Play the EMF's drawing commands onto the given DC.
    */
   bool Playback(HDC aDeviceContext, const RECT* aRect);
 
   /**
    * Called to generate the EMF file once a consumer has finished drawing to