diff --git a/dom/base/nsDocument.cpp b/dom/base/nsDocument.cpp
index 26c1025646..09146b2fe2 100644
--- a/dom/base/nsDocument.cpp
+++ b/dom/base/nsDocument.cpp
@@ -995,7 +995,8 @@ nsExternalResourceMap::PendingLoad::OnDataAvailable(nsIRequest* aRequest,
                                                     uint64_t aOffset,
                                                     uint32_t aCount)
 {
-  NS_PRECONDITION(mTargetListener, "Shouldn't be getting called!");
+  // mTargetListener might be null if SetupViewer or AddExternalResource failed.
+  NS_ENSURE_TRUE(mTargetListener, NS_ERROR_FAILURE);
   if (mDisplayDocument->ExternalResourceMap().HaveShutDown()) {
     return NS_BINDING_ABORTED;
   }