--- a/layout/printing/nsPrintEngine.cpp
+++ b/layout/printing/nsPrintEngine.cpp
@@ -1678,17 +1678,26 @@ nsPrintEngine::ReconstructAndReflow(bool
   }
   return NS_OK;
 }
 
 //-------------------------------------------------------
 nsresult
 nsPrintEngine::SetupToPrintContent()
 {
-  if (NS_WARN_IF(!mPrt)) {
+  // This method may be called while DoCommonPrint() initializes the instance
+  // when its script blocker goes out of scope.  In such case, this cannot do
+  // its job as expected because some objects in mPrt have not been initialized
+  // yet but they are necessary.
+  // Note: it shouldn't be possible for mPrt->mPrintObject to be null; we
+  // just check it for good measure, as we check its owner & members.
+  if (NS_WARN_IF(!mPrt) ||
+      NS_WARN_IF(!mPrt->mPrintObject) ||
+      NS_WARN_IF(!mPrt->mPrintObject->mPresShell) ||
+      NS_WARN_IF(!mPrt->mPrintObject->mPresContext)) {
     return NS_ERROR_FAILURE;
   }
 
   bool didReconstruction = false;
 
   // This method works with mPrt->mPrintObject.  So, we need to guarantee that
   // it won't be deleted in this method.  We achieve this by holding a strong
   // local reference to mPrt, which in turn keeps mPrintObject alive.