--- a/layout/printing/nsPrintEngine.cpp
+++ b/layout/printing/nsPrintEngine.cpp
@@ -326,16 +326,25 @@ nsPrintEngine::InstallPrintPreviewListen
 //----------------------------------------------------------------------
 nsresult 
 nsPrintEngine::GetSeqFrameAndCountPagesInternal(nsPrintObject*  aPO,
                                                 nsIFrame*&    aSeqFrame,
                                                 int32_t&      aCount)
 {
   NS_ENSURE_ARG_POINTER(aPO);
 
+  // This is sometimes incorrectly called before the pres shell has been created
+  // (bug 1141756). MOZ_DIAGNOSTIC_ASSERT so we'll still see the crash in
+  // Nightly/Aurora in case the other patch fixes this.
+  if (!aPO->mPresShell) {
+    MOZ_DIAGNOSTIC_ASSERT(false,
+                          "GetSeqFrameAndCountPages needs a non-null pres shell");
+    return NS_ERROR_FAILURE;
+  }
+
   // Finds the SimplePageSequencer frame
   nsIPageSequenceFrame* seqFrame = aPO->mPresShell->GetPageSequenceFrame();
   aSeqFrame = do_QueryFrame(seqFrame);
   if (!aSeqFrame) {
     return NS_ERROR_FAILURE;
   }
 
   // count the total number of pages